Is there a way to get the function parameter names of a function dynamically?
Let’s say my function looks like this:
function doSomething(param1, param2, .... paramN){
// fill an array with the parameter name and value
// some other code
}
Now, how would I get a list of the parameter names and their values into an array from inside the function?
This question is related to
javascript
reflection
function-parameter
The answer is
Solution that is less error prone to spaces and comments would be:
var fn = function(/* whoa) */ hi, you){};
fn.toString()
.replace(/((\/\/.*$)|(\/\*[\s\S]*?\*\/)|(\s))/mg,'')
.match(/^function\s*[^\(]*\(\s*([^\)]*)\)/m)[1]
.split(/,/)
["hi", "you"]
I've tried doing this before, but never found a praticial way to get it done. I ended up passing in an object instead and then looping through it.
//define like
function test(args) {
for(var item in args) {
alert(item);
alert(args[item]);
}
}
//then used like
test({
name:"Joe",
age:40,
admin:bool
});
It's pretty easy.
At the first there is a deprecated arguments.callee — a reference to called function.
At the second if you have a reference to your function you can easily get their textual representation.
At the third if you calling your function as constructor you can also have a link via yourObject.constructor.
NB: The first solution deprecated so if you can't to not use it you must also think about your app architecture.
If you don't need exact variable names just use inside a function internal variable arguments without any magic.
All of them going to call toString and replace with re so we can create a helper:
// getting names of declared parameters
var getFunctionParams = function (func) {
return String(func).replace(/[^\(]+\(([^\)]*)\).*/m, '$1');
}
Some examples:
// Solution 1. deprecated! don't use it!
var myPrivateFunction = function SomeFuncName (foo, bar, buz) {
console.log(getFunctionParams(arguments.callee));
};
myPrivateFunction (1, 2);
// Solution 2.
var myFunction = function someFunc (foo, bar, buz) {
// some code
};
var params = getFunctionParams(myFunction);
console.log(params);
// Solution 3.
var cls = function SuperKewlClass (foo, bar, buz) {
// some code
};
var inst = new cls();
var params = getFunctionParams(inst.constructor);
console.log(params);
Enjoy with JS!
UPD: Jack Allan was provided a little bit better solution actually. GJ Jack!
function getArgs(args) {
var argsObj = {};
var argList = /\(([^)]*)/.exec(args.callee)[1];
var argCnt = 0;
var tokens;
while (tokens = /\s*([^,]+)/g.exec(argList)) {
argsObj[tokens[1]] = args[argCnt++];
}
return argsObj;
}
Here's one way:
// Utility function to extract arg name-value pairs
function getArgs(args) {
var argsObj = {};
var argList = /\(([^)]*)/.exec(args.callee)[1];
var argCnt = 0;
var tokens;
var argRe = /\s*([^,]+)/g;
while (tokens = argRe.exec(argList)) {
argsObj[tokens[1]] = args[argCnt++];
}
return argsObj;
}
// Test subject
function add(number1, number2) {
var args = getArgs(arguments);
console.log(args); // ({ number1: 3, number2: 4 })
}
// Invoke test subject
add(3, 4);
Note: This only works on browsers that support arguments.callee.
You can access the argument values passed to a function using the "arguments" property.
function doSomething()
{
var args = doSomething.arguments;
var numArgs = args.length;
for(var i = 0 ; i < numArgs ; i++)
{
console.log("arg " + (i+1) + " = " + args[i]);
//console.log works with firefox + firebug
// you can use an alert to check in other browsers
}
}
doSomething(1, '2', {A:2}, [1,2,3]);
This package uses recast in order to create an AST and then the parameter names are gathered from their, this allows it to support pattern matching, default arguments, arrow functions and other ES6 features.
I don't know if this solution suits your problem, but it lets you redefine whatever function you want, without having to change code that uses it. Existing calls will use positioned params, while the function implementation may use "named params" (a single hash param).
I thought that you will anyway modify existing function definitions so, why not having a factory function that makes just what you want:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<script type="text/javascript">
var withNamedParams = function(params, lambda) {
return function() {
var named = {};
var max = arguments.length;
for (var i=0; i<max; i++) {
named[params[i]] = arguments[i];
}
return lambda(named);
};
};
var foo = withNamedParams(["a", "b", "c"], function(params) {
for (var param in params) {
alert(param + ": " + params[param]);
}
});
foo(1, 2, 3);
</script>
</head>
<body>
</body>
</html>
Hope it helps.
function parameter string value image dynamically from JSON. Since item.product_image2 is a URL string, you need to put it in quotes when you call changeImage inside parameter.
My Function Onclick
items+='<img src='+item.product_image1+' id="saleDetailDivGetImg">';
items+="<img src="+item.product_image2+" onclick='changeImage(\""+item.product_image2+"\");'>";
My Function
<script type="text/javascript">
function changeImage(img)
{
document.getElementById("saleDetailDivGetImg").src=img;
alert(img);
}
</script>
Try Manually:
function something(arg1, arg2) {
console.log ( arg1 + arg2 );
}
Here is an updated solution that attempts to address all the edge cases mentioned above in a compact way:
function $args(func) {
return (func + '')
.replace(/[/][/].*$/mg,'') // strip single-line comments
.replace(/\s+/g, '') // strip white space
.replace(/[/][*][^/*]*[*][/]/g, '') // strip multi-line comments
.split('){', 1)[0].replace(/^[^(]*[(]/, '') // extract the parameters
.replace(/=[^,]+/g, '') // strip any ES6 defaults
.split(',').filter(Boolean); // split & filter [""]
}
Abbreviated test output (full test cases are attached below):
'function (a,b,c)...' // returns ["a","b","c"]
'function ()...' // returns []
'function named(a, b, c) ...' // returns ["a","b","c"]
'function (a /* = 1 */, b /* = true */) ...' // returns ["a","b"]
'function fprintf(handle, fmt /*, ...*/) ...' // returns ["handle","fmt"]
'function( a, b = 1, c )...' // returns ["a","b","c"]
'function (a=4*(5/3), b) ...' // returns ["a","b"]
'function (a, // single-line comment xjunk) ...' // returns ["a","b"]
'function (a /* fooled you...' // returns ["a","b"]
'function (a /* function() yes */, \n /* no, */b)/* omg! */...' // returns ["a","b"]
'function ( A, b \n,c ,d \n ) \n ...' // returns ["A","b","c","d"]
'function (a,b)...' // returns ["a","b"]
'function $args(func) ...' // returns ["func"]
'null...' // returns ["null"]
'function Object() ...' // returns []
function $args(func) { _x000D_
return (func + '')_x000D_
.replace(/[/][/].*$/mg,'') // strip single-line comments_x000D_
.replace(/\s+/g, '') // strip white space_x000D_
.replace(/[/][*][^/*]*[*][/]/g, '') // strip multi-line comments _x000D_
.split('){', 1)[0].replace(/^[^(]*[(]/, '') // extract the parameters _x000D_
.replace(/=[^,]+/g, '') // strip any ES6 defaults _x000D_
.split(',').filter(Boolean); // split & filter [""]_x000D_
} _x000D_
_x000D_
// test cases _x000D_
document.getElementById('console_info').innerHTML = (_x000D_
[ _x000D_
// formatting -- typical _x000D_
function(a,b,c){}, _x000D_
function(){}, _x000D_
function named(a, b, c) { _x000D_
/* multiline body */ _x000D_
}, _x000D_
_x000D_
// default values -- conventional _x000D_
function(a /* = 1 */, b /* = true */) { a = a||1; b=b||true; }, _x000D_
function fprintf(handle, fmt /*, ...*/) { }, _x000D_
_x000D_
// default values -- ES6 _x000D_
"function( a, b = 1, c ){}", _x000D_
"function (a=4*(5/3), b) {}", _x000D_
_x000D_
// embedded comments -- sardonic _x000D_
function(a, // single-line comment xjunk) {}_x000D_
b //,c,d_x000D_
) // single-line comment_x000D_
{}, _x000D_
function(a /* fooled you{*/,b){}, _x000D_
function /* are you kidding me? (){} */(a /* function() yes */, _x000D_
/* no, */b)/* omg! */{/*}}*/}, _x000D_
_x000D_
// formatting -- sardonic _x000D_
function ( A, b _x000D_
,c ,d _x000D_
) _x000D_
{ _x000D_
}, _x000D_
_x000D_
// by reference _x000D_
this.jQuery || function (a,b){return new e.fn.init(a,b,h)},_x000D_
$args, _x000D_
_x000D_
// inadvertent non-function values _x000D_
null, _x000D_
Object _x000D_
].map(function(f) {_x000D_
var abbr = (f + '').replace(/\n/g, '\\n').replace(/\s+|[{]+$/g, ' ').split("{", 1)[0] + "...";_x000D_
return " '" + abbr + "' // returns " + JSON.stringify($args(f));_x000D_
}).join("\n") + "\n"); // output for copy and paste as a markdown snippet<pre id='console_info'></pre>I have modified the version taken from AngularJS that implements a dependency injection mechanism to work without Angular. I have also updated the STRIP_COMMENTS regex to work with ECMA6, so it supports things like default values in the signature.
var FN_ARGS = /^function\s*[^\(]*\(\s*([^\)]*)\)/m;_x000D_
var FN_ARG_SPLIT = /,/;_x000D_
var FN_ARG = /^\s*(_?)(.+?)\1\s*$/;_x000D_
var STRIP_COMMENTS = /(\/\/.*$)|(\/\*[\s\S]*?\*\/)|(\s*=[^,\)]*(('(?:\\'|[^'\r\n])*')|("(?:\\"|[^"\r\n])*"))|(\s*=[^,\)]*))/mg;_x000D_
_x000D_
function annotate(fn) {_x000D_
var $inject,_x000D_
fnText,_x000D_
argDecl,_x000D_
last;_x000D_
_x000D_
if (typeof fn == 'function') {_x000D_
if (!($inject = fn.$inject)) {_x000D_
$inject = [];_x000D_
fnText = fn.toString().replace(STRIP_COMMENTS, '');_x000D_
argDecl = fnText.match(FN_ARGS);_x000D_
argDecl[1].split(FN_ARG_SPLIT).forEach(function(arg) {_x000D_
arg.replace(FN_ARG, function(all, underscore, name) {_x000D_
$inject.push(name);_x000D_
});_x000D_
});_x000D_
fn.$inject = $inject;_x000D_
}_x000D_
} else {_x000D_
throw Error("not a function")_x000D_
}_x000D_
return $inject;_x000D_
}_x000D_
_x000D_
console.log("function(a, b)",annotate(function(a, b) {_x000D_
console.log(a, b, c, d)_x000D_
}))_x000D_
console.log("function(a, b = 0, /*c,*/ d)",annotate(function(a, b = 0, /*c,*/ d) {_x000D_
console.log(a, b, c, d)_x000D_
}))_x000D_
annotate({})You can also use "esprima" parser to avoid many issues with comments, whitespace and other things inside parameters list.
function getParameters(yourFunction) {
var i,
// safetyValve is necessary, because sole "function () {...}"
// is not a valid syntax
parsed = esprima.parse("safetyValve = " + yourFunction.toString()),
params = parsed.body[0].expression.right.params,
ret = [];
for (i = 0; i < params.length; i += 1) {
// Handle default params. Exe: function defaults(a = 0,b = 2,c = 3){}
if (params[i].type == 'AssignmentPattern') {
ret.push(params[i].left.name)
} else {
ret.push(params[i].name);
}
}
return ret;
}
It works even with code like this:
getParameters(function (hello /*, foo ),* /bar* { */,world) {}); // ["hello", "world"]
i would likke to suggest solution which supports arrow functions like i used this article for basic regular expression and https://davidwalsh.name/javascript-arguments and added arrow functions support
(arg1,arg2) => {}
or
arg => {}
function getArgs(func) {
if(func.length === 0){
return []
}
let string = func.toString();
let args;
// First match everything inside the function argument parens. like `function (arg1,arg2) {}` or `async function(arg1,arg2) {}
args = string.match(/(?:async|function)\s*.*?\(([^)]*)\)/)?.[1] ||
// arrow functions with multiple arguments like `(arg1,arg2) => {}`
string.match(/^\s*\(([^)]*)\)\s*=>/)?.[1] ||
// arrow functions with single argument without parens like `arg => {}`
string.match(/^\s*([^=]*)=>/)?.[1]
// Split the arguments string into an array comma delimited.
return args.split(',').map(function(arg) {
// Ensure no inline comments are parsed and trim the whitespace.
return arg.replace(/\/\*.*\*\//, '').trim();
}).filter(function(arg) {
// Ensure no undefined values are added.
return arg;
});
}
(function(a,b,c){}).toString().replace(/.*\(|\).*/ig,"").split(',')
=> [ "a", "b", "c" ]
Below is the code taken from AngularJS which uses the technique for its dependency injection mechanism.
And here is an explanation of it taken from http://docs.angularjs.org/tutorial/step_05
Angular's dependency injector provides services to your controller when the controller is being constructed. The dependency injector also takes care of creating any transitive dependencies the service may have (services often depend upon other services).
Note that the names of arguments are significant, because the injector uses these to look up the dependencies.
/**
* @ngdoc overview
* @name AUTO
* @description
*
* Implicit module which gets automatically added to each {@link AUTO.$injector $injector}.
*/
var FN_ARGS = /^function\s*[^\(]*\(\s*([^\)]*)\)/m;
var FN_ARG_SPLIT = /,/;
var FN_ARG = /^\s*(_?)(.+?)\1\s*$/;
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
function annotate(fn) {
var $inject,
fnText,
argDecl,
last;
if (typeof fn == 'function') {
if (!($inject = fn.$inject)) {
$inject = [];
fnText = fn.toString().replace(STRIP_COMMENTS, '');
argDecl = fnText.match(FN_ARGS);
forEach(argDecl[1].split(FN_ARG_SPLIT), function(arg){
arg.replace(FN_ARG, function(all, underscore, name){
$inject.push(name);
});
});
fn.$inject = $inject;
}
} else if (isArray(fn)) {
last = fn.length - 1;
assertArgFn(fn[last], 'fn')
$inject = fn.slice(0, last);
} else {
assertArgFn(fn, 'fn', true);
}
return $inject;
}
The proper way to do this is to use a JS parser. Here is an example using acorn.
const acorn = require('acorn');
function f(a, b, c) {
// ...
}
const argNames = acorn.parse(f).body[0].params.map(x => x.name);
console.log(argNames); // Output: [ 'a', 'b', 'c' ]
The code here finds the names of the three (formal) parameters of the function f. It does so by feeding f into acorn.parse().
I have read most of the answers here, and I would like to add my one-liner.
new RegExp('(?:'+Function.name+'\\s*|^)\\((.*?)\\)').exec(Function.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '')
or
function getParameters(func) {
return new RegExp('(?:'+func.name+'\\s*|^)\\s*\\((.*?)\\)').exec(func.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '');
}
or for a one-liner function in ECMA6
var getParameters = func => new RegExp('(?:'+func.name+'\\s*|^)\\s*\\((.*?)\\)').exec(func.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '');
__
Let's say you have a function
function foo(abc, def, ghi, jkl) {
//code
}
The below code will return "abc,def,ghi,jkl"
That code will also work with the setup of a function that Camilo Martin gave:
function ( A, b
,c ,d
){}
Also with Bubersson's comment on Jack Allan's answer:
function(a /* fooled you)*/,b){}
__
Explanation
new RegExp('(?:'+Function.name+'\\s*|^)\\s*\\((.*?)\\)')
This creates a Regular Expression with the new RegExp('(?:'+Function.name+'\\s*|^)\\s*\\((.*?)\\)'). I have to use new RegExp because I am injecting a variable (Function.name, the name of the function being targeted) into the RegExp.
Example If the function name is "foo" (function foo()), the RegExp will be /foo\s*\((.*?)\)/.
Function.toString().replace(/\n/g, '')
Then it converts the entire function into a string, and removes all newlines. Removing newlines helps with the function setup Camilo Martin gave.
.exec(...)[1]
This is the RegExp.prototype.exec function. It basically matches the Regular Exponent (new RegExp()) into the String (Function.toString()). Then the [1] will return the first Capture Group found in the Regular Exponent ((.*?)).
.replace(/\/\*.*?\*\//g, '').replace(/ /g, '')
This will remove every comment inside /* and */, and remove all spaces.
This also now supports reading and understanding arrow (=>) functions, such as f = (a, b) => void 0;, in which Function.toString() would return (a, b) => void 0 instead of the normal function's function f(a, b) { return void 0; }. The original regular expression would have thrown an error in its confusion, but is now accounted for.
The change was from new RegExp(Function.name+'\\s*\\((.*?)\\)') (/Function\s*\((.*?)\)/) to new RegExp('(?:'+Function.name+'\\s*|^)\\((.*?)\\)') (/(?:Function\s*|^)\((.*?)\)/)
If you want to make all the parameters into an Array instead of a String separated by commas, at the end just add .split(',').
As this has not yet been mentioned, if you are using Typescript you can emit meta-data when using Decorators which will allow you to get the parameter names and types.
Metadata will only be emitted if the class/function/prop has a decorator on it.
It doesn't matter which decorator.
This feature can be enabled by setting emitDecoratorMetadata to true inside tsconfig.json
{
"compilerOptions": {
"emitDecoratorMetadata": true
}
}
As the metadata is still an early proposal the reflect-metadata package must be installed or Reflect.getMetadata will not be defined.
npm install reflect-metadata
You can use it as follows:
const AnyDecorator = () : MethodDecorator => {
return target => { }
}
class Person{
@AnyDecorator()
sayHello(other: Person){}
}
const instance = new Person();
const funcType = Reflect.getMetadata('design:type', instance.sayHello);
const funcParams = Reflect.getMetadata('design:paramtypes', instance.sayHello);
In newer versions of Angular for instance this is used to determine what to inject -> https://stackoverflow.com/a/53041387/1087372
I don't know how to get a list of the parameters but you can do this to get how many it expects.
alert(doSomething.length);
Whatever the solution, it must not break on wierd functions, whose toString() looks just as wierd:
function ( A, b
,c ,d
){}
Also, why use complex regular expressions? This can be done like:
function getArguments(f) {
return f.toString().split(')',1)[0].replace(/\s/g,'').substr(9).split(',');
}
This works everywhere with every function, and the only regex is whitespace removal that doesn't even process the whole string due to the .split trick.
I know this is an old question, but beginners have been copypasting this around as if this was good practice in any code. Most of the time, having to parse a function's string representation to use its parameter names just hides a flaw in the code's logic.
Parameters of a function are actually stored in an array-like object called arguments, where the first argument is arguments[0], the second is arguments[1] and so on. Writing parameter names in the parentheses can be seen as a shorthand syntax. This:
function doSomething(foo, bar) {
console.log("does something");
}
...is the same as:
function doSomething() {
var foo = arguments[0];
var bar = arguments[1];
console.log("does something");
}
The variables themselves are stored in the function's scope, not as properties in an object. There is no way to retrieve the parameter name through code as it is merely a symbol representing the variable in human-language.
I always considered the string representation of a function as a tool for debugging purposes, especially because of this arguments array-like object. You are not required to give names to the arguments in the first place. If you try parsing a stringified function, it doesn't actually tell you about extra unnamed parameters it might take.
Here's an even worse and more common situation. If a function has more than 3 or 4 arguments, it might be logical to pass it an object instead, which is easier to work with.
function saySomething(obj) {
if(obj.message) console.log((obj.sender || "Anon") + ": " + obj.message);
}
saySomething({sender: "user123", message: "Hello world"});
In this case, the function itself will be able to read through the object it receives and look for its properties and get both their names and values, but trying to parse the string representation of the function would only give you "obj" for parameters, which isn't useful at all.
//See this:
// global var, naming bB
var bB = 5;
// Dependency Injection cokntroller
var a = function(str, fn) {
//stringify function body
var fnStr = fn.toString();
// Key: get form args to string
var args = fnStr.match(/function\s*\((.*?)\)/);
//
console.log(args);
// if the form arg is 'bB', then exec it, otherwise, do nothing
for (var i = 0; i < args.length; i++) {
if(args[i] == 'bB') {
fn(bB);
}
}
}
// will do nothing
a('sdfdfdfs,', function(some){
alert(some)
});
// will alert 5
a('sdfdsdsfdfsdfdsf,', function(bB){
alert(bB)
});
// see, this shows you how to get function args in string
Taking the answer from @jack-allan I modified the function slightly to allow ES6 default properties such as:
function( a, b = 1, c ){};
to still return [ 'a', 'b' ]
/**
* Get the keys of the paramaters of a function.
*
* @param {function} method Function to get parameter keys for
* @return {array}
*/
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
var ARGUMENT_NAMES = /(?:^|,)\s*([^\s,=]+)/g;
function getFunctionParameters ( func ) {
var fnStr = func.toString().replace(STRIP_COMMENTS, '');
var argsList = fnStr.slice(fnStr.indexOf('(')+1, fnStr.indexOf(')'));
var result = argsList.match( ARGUMENT_NAMES );
if(result === null) {
return [];
}
else {
var stripped = [];
for ( var i = 0; i < result.length; i++ ) {
stripped.push( result[i].replace(/[\s,]/g, '') );
}
return stripped;
}
}
Note: if you want to use ES6 parameter destructuring with the top solution add the following line.
if (result[0] === '{' && result[result.length - 1 === '}']) result = result.slice(1, -1)
Ok so an old question with plenty of adequate answers. here is my offering that does not use regex, except for the menial task of stripping whitespace . (I should note that the "strips_comments" function actually spaces them out, rather than physically remove them. that's because i use it elsewhere and for various reasons need the locations of the original non comment tokens to stay intact)
It's a fairly lengthy block of code as this pasting includes a mini test framework.
function do_tests(func) {
if (typeof func !== 'function') return true;
switch (typeof func.tests) {
case 'undefined' : return true;
case 'object' :
for (var k in func.tests) {
var test = func.tests[k];
if (typeof test==='function') {
var result = test(func);
if (result===false) {
console.log(test.name,'for',func.name,'failed');
return false;
}
}
}
return true;
case 'function' :
return func.tests(func);
}
return true;
}
function strip_comments(src) {
var spaces=(s)=>{
switch (s) {
case 0 : return '';
case 1 : return ' ';
case 2 : return ' ';
default :
return Array(s+1).join(' ');
}
};
var c1 = src.indexOf ('/*'),
c2 = src.indexOf ('//'),
eol;
var out = "";
var killc2 = () => {
out += src.substr(0,c2);
eol = src.indexOf('\n',c2);
if (eol>=0) {
src = spaces(eol-c2)+'\n'+src.substr(eol+1);
} else {
src = spaces(src.length-c2);
return true;
}
return false;
};
while ((c1>=0) || (c2>=0)) {
if (c1>=0) {
// c1 is a hit
if ( (c1<c2) || (c2<0) ) {
// and it beats c2
out += src.substr(0,c1);
eol = src.indexOf('*/',c1+2);
if (eol>=0) {
src = spaces((eol-c1)+2)+src.substr(eol+2);
} else {
src = spaces(src.length-c1);
break;
}
} else {
if (c2 >=0) {
// c2 is a hit and it beats c1
if (killc2()) break;
}
}
} else {
if (c2>=0) {
// c2 is a hit, c1 is a miss.
if (killc2()) break;
} else {
// both c1 & c2 are a miss
break;
}
}
c1 = src.indexOf ('/*');
c2 = src.indexOf ('//');
}
return out + src;
}
function function_args(fn) {
var src = strip_comments(fn.toString());
var names=src.split(')')[0].replace(/\s/g,'').split('(')[1].split(',');
return names;
}
function_args.tests = [
function test1 () {
function/*al programmers will sometimes*/strip_comments_tester/* because some comments are annoying*/(
/*see this---(((*/ src//)) it's an annoying comment does not help anyone understand if the
,code,//really does
/**/sucks ,much /*?*/)/*who would put "comment\" about a function like (this) { comment } here?*/{
}
var data = function_args(strip_comments_tester);
return ( (data.length==4) &&
(data[0]=='src') &&
(data[1]=='code') &&
(data[2]=='sucks') &&
(data[3]=='much') );
}
];
do_tests(function_args);
A lot of the answers on here use regexes, this is fine but it doesn't handle new additions to the language too well (like arrow functions and classes). Also of note is that if you use any of these functions on minified code it's going to go . It will use whatever the minified name is. Angular gets around this by allowing you to pass in an ordered array of strings that matches the order of the arguments when registering them with the DI container. So on with the solution:
var esprima = require('esprima');
var _ = require('lodash');
const parseFunctionArguments = (func) => {
// allows us to access properties that may or may not exist without throwing
// TypeError: Cannot set property 'x' of undefined
const maybe = (x) => (x || {});
// handle conversion to string and then to JSON AST
const functionAsString = func.toString();
const tree = esprima.parse(functionAsString);
console.log(JSON.stringify(tree, null, 4))
// We need to figure out where the main params are. Stupid arrow functions
const isArrowExpression = (maybe(_.first(tree.body)).type == 'ExpressionStatement');
const params = isArrowExpression ? maybe(maybe(_.first(tree.body)).expression).params
: maybe(_.first(tree.body)).params;
// extract out the param names from the JSON AST
return _.map(params, 'name');
};
This handles the original parse issue and a few more function types (e.g. arrow functions). Here's an idea of what it can and can't handle as is:
// I usually use mocha as the test runner and chai as the assertion library
describe('Extracts argument names from function signature. ', () => {
const test = (func) => {
const expectation = ['it', 'parses', 'me'];
const result = parseFunctionArguments(toBeParsed);
result.should.equal(expectation);
}
it('Parses a function declaration.', () => {
function toBeParsed(it, parses, me){};
test(toBeParsed);
});
it('Parses a functional expression.', () => {
const toBeParsed = function(it, parses, me){};
test(toBeParsed);
});
it('Parses an arrow function', () => {
const toBeParsed = (it, parses, me) => {};
test(toBeParsed);
});
// ================= cases not currently handled ========================
// It blows up on this type of messing. TBH if you do this it deserves to
// fail On a tech note the params are pulled down in the function similar
// to how destructuring is handled by the ast.
it('Parses complex default params', () => {
function toBeParsed(it=4*(5/3), parses, me) {}
test(toBeParsed);
});
// This passes back ['_ref'] as the params of the function. The _ref is a
// pointer to an VariableDeclarator where the ? happens.
it('Parses object destructuring param definitions.' () => {
function toBeParsed ({it, parses, me}){}
test(toBeParsed);
});
it('Parses object destructuring param definitions.' () => {
function toBeParsed ([it, parses, me]){}
test(toBeParsed);
});
// Classes while similar from an end result point of view to function
// declarations are handled completely differently in the JS AST.
it('Parses a class constructor when passed through', () => {
class ToBeParsed {
constructor(it, parses, me) {}
}
test(ToBeParsed);
});
});
Depending on what you want to use it for ES6 Proxies and destructuring may be your best bet. For example if you wanted to use it for dependency injection (using the names of the params) then you can do it as follows:
class GuiceJs {
constructor() {
this.modules = {}
}
resolve(name) {
return this.getInjector()(this.modules[name]);
}
addModule(name, module) {
this.modules[name] = module;
}
getInjector() {
var container = this;
return (klass) => {
console.log(klass);
var paramParser = new Proxy({}, {
// The `get` handler is invoked whenever a get-call for
// `injector.*` is made. We make a call to an external service
// to actually hand back in the configured service. The proxy
// allows us to bypass parsing the function params using
// taditional regex or even the newer parser.
get: (target, name) => container.resolve(name),
// You shouldn't be able to set values on the injector.
set: (target, name, value) => {
throw new Error(`Don't try to set ${name}! `);
}
})
return new klass(paramParser);
}
}
}
It's not the most advanced resolver out there but it gives an idea of how you can use a Proxy to handle it if you want to use args parser for simple DI. There is however one slight caveat in this approach. We need to use destructuring assignments instead of normal params. When we pass in the injector proxy the destructuring is the same as calling the getter on the object.
class App {
constructor({tweeter, timeline}) {
this.tweeter = tweeter;
this.timeline = timeline;
}
}
class HttpClient {}
class TwitterApi {
constructor({client}) {
this.client = client;
}
}
class Timeline {
constructor({api}) {
this.api = api;
}
}
class Tweeter {
constructor({api}) {
this.api = api;
}
}
// Ok so now for the business end of the injector!
const di = new GuiceJs();
di.addModule('client', HttpClient);
di.addModule('api', TwitterApi);
di.addModule('tweeter', Tweeter);
di.addModule('timeline', Timeline);
di.addModule('app', App);
var app = di.resolve('app');
console.log(JSON.stringify(app, null, 4));
This outputs the following:
{
"tweeter": {
"api": {
"client": {}
}
},
"timeline": {
"api": {
"client": {}
}
}
}
Its wired up the entire application. The best bit is that the app is easy to test (you can just instantiate each class and pass in mocks/stubs/etc). Also if you need to swap out implementations, you can do that from a single place. All this is possible because of JS Proxy objects.
Note: There is a lot of work that would need to be done to this before it would be ready for production use but it does give an idea of what it would look like.
It's a bit late in the answer but it may help others who are thinking of the same thing.
How I typically do it:
function name(arg1, arg2){
var args = arguments; // array: [arg1, arg2]
var objecArgOne = args[0].one;
}
name({one: "1", two: "2"}, "string");
You can even ref the args by the functions name like:
name.arguments;
Hope this helps!
The following function will return an array of the parameter names of any function passed in.
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
var ARGUMENT_NAMES = /([^\s,]+)/g;
function getParamNames(func) {
var fnStr = func.toString().replace(STRIP_COMMENTS, '');
var result = fnStr.slice(fnStr.indexOf('(')+1, fnStr.indexOf(')')).match(ARGUMENT_NAMES);
if(result === null)
result = [];
return result;
}
Example usage:
getParamNames(getParamNames) // returns ['func']
getParamNames(function (a,b,c,d){}) // returns ['a','b','c','d']
getParamNames(function (a,/*b,c,*/d){}) // returns ['a','d']
getParamNames(function (){}) // returns []
Edit:
With the invent of ES6 this function can be tripped up by default parameters. Here is a quick hack which should work in most cases:
var STRIP_COMMENTS = /(\/\/.*$)|(\/\*[\s\S]*?\*\/)|(\s*=[^,\)]*(('(?:\\'|[^'\r\n])*')|("(?:\\"|[^"\r\n])*"))|(\s*=[^,\)]*))/mg;
I say most cases because there are some things that will trip it up
function (a=4*(5/3), b) {} // returns ['a']
Edit: I also note vikasde wants the parameter values in an array also. This is already provided in a local variable named arguments.
excerpt from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/arguments:
The arguments object is not an Array. It is similar to an Array, but does not have any Array properties except length. For example, it does not have the pop method. However it can be converted to a real Array:
var args = Array.prototype.slice.call(arguments);
If Array generics are available, one can use the following instead:
var args = Array.slice(arguments);
Since JavaScript is a scripting language, I feel that its introspection should support getting function parameter names. Punting on that functionality is a violation of first principles, so I decided to explore the issue further.
That led me to this question but no built-in solutions. Which led me to this answer which explains that arguments is only deprecated outside the function, so we can no longer use myFunction.arguments or we get:
TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them
Time to roll up our sleeves and get to work:
? Retrieving function parameters requires a parser because complex expressions like 4*(5/3) can be used as default values. So Gaafar's answer or James Drew's answer are so far the best approaches.
I tried the babylon and esprima parsers but unfortunately they can't parse standalone anonymous functions, as pointed out in Mateusz Charytoniuk's answer. I figured out another workaround though by surrounding the code in parentheses, so as not to change the logic:
const ast = parser.parse("(\n" + func.toString() + "\n)")
The newlines prevent issues with // (single-line comments).
? If a parser is not available, the next-best option is to use a tried-and-true technique like Angular.js's dependency injector regular expressions. I combined a functional version of Lambder's answer with humbletim's answer and added an optional ARROW boolean for controlling whether ES6 fat arrow functions are allowed by the regular expressions.
Here are two solutions I put together. Note that these have no logic to detect whether a function has valid syntax, they only extract the arguments. This is generally ok since we usually pass parsed functions to getArguments() so their syntax is already valid.
I will try to curate these solutions as best I can, but without effort from the JavaScript maintainers, this will remain an open problem.
Node.js version (not runnable until StackOverflow supports Node.js):
const parserName = 'babylon';_x000D_
// const parserName = 'esprima';_x000D_
const parser = require(parserName);_x000D_
_x000D_
function getArguments(func) {_x000D_
const maybe = function (x) {_x000D_
return x || {}; // optionals support_x000D_
}_x000D_
_x000D_
try {_x000D_
const ast = parser.parse("(\n" + func.toString() + "\n)");_x000D_
const program = parserName == 'babylon' ? ast.program : ast;_x000D_
_x000D_
return program_x000D_
.body[0]_x000D_
.expression_x000D_
.params_x000D_
.map(function(node) {_x000D_
return node.name || maybe(node.left).name || '...' + maybe(node.argument).name;_x000D_
});_x000D_
} catch (e) {_x000D_
return []; // could also return null_x000D_
}_x000D_
};_x000D_
_x000D_
////////// TESTS //////////_x000D_
_x000D_
function logArgs(func) {_x000D_
let object = {};_x000D_
_x000D_
object[func] = getArguments(func);_x000D_
_x000D_
console.log(object);_x000D_
// console.log(/*JSON.stringify(*/getArguments(func)/*)*/);_x000D_
}_x000D_
_x000D_
console.log('');_x000D_
console.log('////////// MISC //////////');_x000D_
_x000D_
logArgs((a, b) => {});_x000D_
logArgs((a, b = 1) => {});_x000D_
logArgs((a, b, ...args) => {});_x000D_
logArgs(function(a, b, ...args) {});_x000D_
logArgs(function(a, b = 1, c = 4 * (5 / 3), d = 2) {});_x000D_
logArgs(async function(a, b, ...args) {});_x000D_
logArgs(function async(a, b, ...args) {});_x000D_
_x000D_
console.log('');_x000D_
console.log('////////// FUNCTIONS //////////');_x000D_
_x000D_
logArgs(function(a, b, c) {});_x000D_
logArgs(function() {});_x000D_
logArgs(function named(a, b, c) {});_x000D_
logArgs(function(a /* = 1 */, b /* = true */) {});_x000D_
logArgs(function fprintf(handle, fmt /*, ...*/) {});_x000D_
logArgs(function(a, b = 1, c) {});_x000D_
logArgs(function(a = 4 * (5 / 3), b) {});_x000D_
// logArgs(function (a, // single-line comment xjunk) {});_x000D_
// logArgs(function (a /* fooled you {});_x000D_
// logArgs(function (a /* function() yes */, \n /* no, */b)/* omg! */ {});_x000D_
// logArgs(function ( A, b \n,c ,d \n ) \n {});_x000D_
logArgs(function(a, b) {});_x000D_
logArgs(function $args(func) {});_x000D_
logArgs(null);_x000D_
logArgs(function Object() {});_x000D_
_x000D_
console.log('');_x000D_
console.log('////////// STRINGS //////////');_x000D_
_x000D_
logArgs('function (a,b,c) {}');_x000D_
logArgs('function () {}');_x000D_
logArgs('function named(a, b, c) {}');_x000D_
logArgs('function (a /* = 1 */, b /* = true */) {}');_x000D_
logArgs('function fprintf(handle, fmt /*, ...*/) {}');_x000D_
logArgs('function( a, b = 1, c ) {}');_x000D_
logArgs('function (a=4*(5/3), b) {}');_x000D_
logArgs('function (a, // single-line comment xjunk) {}');_x000D_
logArgs('function (a /* fooled you {}');_x000D_
logArgs('function (a /* function() yes */, \n /* no, */b)/* omg! */ {}');_x000D_
logArgs('function ( A, b \n,c ,d \n ) \n {}');_x000D_
logArgs('function (a,b) {}');_x000D_
logArgs('function $args(func) {}');_x000D_
logArgs('null');_x000D_
logArgs('function Object() {}');Full working example:
https://repl.it/repls/SandybrownPhonyAngles
Browser version (note that it stops at the first complex default value):
function getArguments(func) {_x000D_
const ARROW = true;_x000D_
const FUNC_ARGS = ARROW ? /^(function)?\s*[^\(]*\(\s*([^\)]*)\)/m : /^(function)\s*[^\(]*\(\s*([^\)]*)\)/m;_x000D_
const FUNC_ARG_SPLIT = /,/;_x000D_
const FUNC_ARG = /^\s*(_?)(.+?)\1\s*$/;_x000D_
const STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;_x000D_
_x000D_
return ((func || '').toString().replace(STRIP_COMMENTS, '').match(FUNC_ARGS) || ['', '', ''])[2]_x000D_
.split(FUNC_ARG_SPLIT)_x000D_
.map(function(arg) {_x000D_
return arg.replace(FUNC_ARG, function(all, underscore, name) {_x000D_
return name.split('=')[0].trim();_x000D_
});_x000D_
})_x000D_
.filter(String);_x000D_
}_x000D_
_x000D_
////////// TESTS //////////_x000D_
_x000D_
function logArgs(func) {_x000D_
let object = {};_x000D_
_x000D_
object[func] = getArguments(func);_x000D_
_x000D_
console.log(object);_x000D_
// console.log(/*JSON.stringify(*/getArguments(func)/*)*/);_x000D_
}_x000D_
_x000D_
console.log('');_x000D_
console.log('////////// MISC //////////');_x000D_
_x000D_
logArgs((a, b) => {});_x000D_
logArgs((a, b = 1) => {});_x000D_
logArgs((a, b, ...args) => {});_x000D_
logArgs(function(a, b, ...args) {});_x000D_
logArgs(function(a, b = 1, c = 4 * (5 / 3), d = 2) {});_x000D_
logArgs(async function(a, b, ...args) {});_x000D_
logArgs(function async(a, b, ...args) {});_x000D_
_x000D_
console.log('');_x000D_
console.log('////////// FUNCTIONS //////////');_x000D_
_x000D_
logArgs(function(a, b, c) {});_x000D_
logArgs(function() {});_x000D_
logArgs(function named(a, b, c) {});_x000D_
logArgs(function(a /* = 1 */, b /* = true */) {});_x000D_
logArgs(function fprintf(handle, fmt /*, ...*/) {});_x000D_
logArgs(function(a, b = 1, c) {});_x000D_
logArgs(function(a = 4 * (5 / 3), b) {});_x000D_
// logArgs(function (a, // single-line comment xjunk) {});_x000D_
// logArgs(function (a /* fooled you {});_x000D_
// logArgs(function (a /* function() yes */, \n /* no, */b)/* omg! */ {});_x000D_
// logArgs(function ( A, b \n,c ,d \n ) \n {});_x000D_
logArgs(function(a, b) {});_x000D_
logArgs(function $args(func) {});_x000D_
logArgs(null);_x000D_
logArgs(function Object() {});_x000D_
_x000D_
console.log('');_x000D_
console.log('////////// STRINGS //////////');_x000D_
_x000D_
logArgs('function (a,b,c) {}');_x000D_
logArgs('function () {}');_x000D_
logArgs('function named(a, b, c) {}');_x000D_
logArgs('function (a /* = 1 */, b /* = true */) {}');_x000D_
logArgs('function fprintf(handle, fmt /*, ...*/) {}');_x000D_
logArgs('function( a, b = 1, c ) {}');_x000D_
logArgs('function (a=4*(5/3), b) {}');_x000D_
logArgs('function (a, // single-line comment xjunk) {}');_x000D_
logArgs('function (a /* fooled you {}');_x000D_
logArgs('function (a /* function() yes */, \n /* no, */b)/* omg! */ {}');_x000D_
logArgs('function ( A, b \n,c ,d \n ) \n {}');_x000D_
logArgs('function (a,b) {}');_x000D_
logArgs('function $args(func) {}');_x000D_
logArgs('null');_x000D_
logArgs('function Object() {}');Full working example:
Wow so many answers already.. Im pretty sure this gets buried. Even so I figured this might be useful for some.
I wasn't fully satisfied with the chosen answers as in ES6 it doesn't work well with default values. And it also does not provide the default value information. I also wanted a lightweight function that does not depend on an external lib.
This function is very useful for debugging purposes, for example: logging called function with its params, default param values and arguments.
I spent some time on this yesterday, cracking the right RegExp to solve this issue and this is what I came up with. It works very well and I'm very pleased with the outcome:
const REGEX_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;_x000D_
const REGEX_FUNCTION_PARAMS = /(?:\s*(?:function\s*[^(]*)?\s*)((?:[^'"]|(?:(?:(['"])(?:(?:.*?[^\\]\2)|\2))))*?)\s*(?=(?:=>)|{)/m_x000D_
const REGEX_PARAMETERS_VALUES = /\s*(\w+)\s*(?:=\s*((?:(?:(['"])(?:\3|(?:.*?[^\\]\3)))((\s*\+\s*)(?:(?:(['"])(?:\6|(?:.*?[^\\]\6)))|(?:[\w$]*)))*)|.*?))?\s*(?:,|$)/gm_x000D_
_x000D_
/**_x000D_
* Retrieve a function's parameter names and default values_x000D_
* Notes:_x000D_
* - parameters with default values will not show up in transpiler code (Babel) because the parameter is removed from the function._x000D_
* - does NOT support inline arrow functions as default values_x000D_
* to clarify: ( name = "string", add = defaultAddFunction ) - is ok_x000D_
* ( name = "string", add = ( a )=> a + 1 ) - is NOT ok_x000D_
* - does NOT support default string value that are appended with a non-standard ( word characters or $ ) variable name_x000D_
* to clarify: ( name = "string" + b ) - is ok_x000D_
* ( name = "string" + $b ) - is ok_x000D_
* ( name = "string" + b + "!" ) - is ok_x000D_
* ( name = "string" + ? ) - is NOT ok_x000D_
* @param {function} func_x000D_
* @returns {Array} - An array of the given function's parameter [key, default value] pairs._x000D_
*/_x000D_
function getParams(func) {_x000D_
_x000D_
let functionAsString = func.toString()_x000D_
let params = []_x000D_
let match_x000D_
functionAsString = functionAsString.replace(REGEX_COMMENTS, '')_x000D_
functionAsString = functionAsString.match(REGEX_FUNCTION_PARAMS)[1]_x000D_
if (functionAsString.charAt(0) === '(') functionAsString = functionAsString.slice(1, -1)_x000D_
while (match = REGEX_PARAMETERS_VALUES.exec(functionAsString)) params.push([match[1], match[2]])_x000D_
return params_x000D_
_x000D_
}_x000D_
_x000D_
_x000D_
_x000D_
// Lets run some tests!_x000D_
_x000D_
var defaultName = 'some name'_x000D_
_x000D_
function test1(param1, param2, param3) { return (param1) => param1 + param2 + param3 }_x000D_
function test2(param1, param2 = 4 * (5 / 3), param3) {}_x000D_
function test3(param1, param2 = "/root/" + defaultName + ".jpeg", param3) {}_x000D_
function test4(param1, param2 = (a) => a + 1) {}_x000D_
_x000D_
console.log(getParams(test1)) _x000D_
console.log(getParams(test2))_x000D_
console.log(getParams(test3))_x000D_
console.log(getParams(test4))_x000D_
_x000D_
// [ [ 'param1', undefined ], [ 'param2', undefined ], [ 'param3', undefined ] ]_x000D_
// [ [ 'param1', undefined ], [ 'param2', '4 * (5 / 3)' ], [ 'param3', undefined ] ]_x000D_
// [ [ 'param1', undefined ], [ 'param2', '"/root/" + defaultName + ".jpeg"' ], [ 'param3', undefined ] ]_x000D_
// [ [ 'param1', undefined ], [ 'param2', '( a' ] ]_x000D_
// --> This last one fails because of the inlined arrow function!_x000D_
_x000D_
_x000D_
var arrowTest1 = (a = 1) => a + 4_x000D_
var arrowTest2 = a => b => a + b_x000D_
var arrowTest3 = (param1 = "/" + defaultName) => { return param1 + '...' }_x000D_
var arrowTest4 = (param1 = "/" + defaultName, param2 = 4, param3 = null) => { () => param3 ? param3 : param2 }_x000D_
_x000D_
console.log(getParams(arrowTest1))_x000D_
console.log(getParams(arrowTest2))_x000D_
console.log(getParams(arrowTest3))_x000D_
console.log(getParams(arrowTest4))_x000D_
_x000D_
// [ [ 'a', '1' ] ]_x000D_
// [ [ 'a', undefined ] ]_x000D_
// [ [ 'param1', '"/" + defaultName' ] ]_x000D_
// [ [ 'param1', '"/" + defaultName' ], [ 'param2', '4' ], [ 'param3', 'null' ] ]_x000D_
_x000D_
_x000D_
console.log(getParams((param1) => param1 + 1))_x000D_
console.log(getParams((param1 = 'default') => { return param1 + '.jpeg' }))_x000D_
_x000D_
// [ [ 'param1', undefined ] ]_x000D_
// [ [ 'param1', '\'default\'' ] ]As you can tell some of the parameter names disappear because the Babel transpiler removes them from the function. If you would run this in the latest NodeJS it works as expected (The commented results are from NodeJS).
Another note, as stated in the comment is that is does not work with inlined arrow functions as a default value. This simply makes it far to complex to extract the values using a RegExp.
Please let me know if this was useful for you! Would love to hear some feedback!
I'll give you a short example below:
function test(arg1,arg2){
var funcStr = test.toString()
var leftIndex = funcStr.indexOf('(');
var rightIndex = funcStr.indexOf(')');
var paramStr = funcStr.substr(leftIndex+1,rightIndex-leftIndex-1);
var params = paramStr.split(',');
for(param of params){
console.log(param); // arg1,arg2
}
}
test();
The answer to this requires 3 steps:
- To get the values of the actual parameters passed to the function (let's call it
argValues). This is straight forward as it will be available asargumentsinside the function. - To get the parameter names from the function signature (let's call it
argNames). This not as easy and requires parsing the function. Instead of doing the complex regex yourself and worrying about edge cases (default parameters, comments, ...), you can use a library like babylon that will parse the function into an abstract syntax tree from which you can obtain the names of parameters. - The last step is to join the 2 arrays together into 1 array that has the name and value of all the parameters.
The code will be like this
const babylon = require("babylon")
function doSomething(a, b, c) {
// get the values of passed argumenst
const argValues = arguments
// get the names of the arguments by parsing the function
const ast = babylon.parse(doSomething.toString())
const argNames = ast.program.body[0].params.map(node => node.name)
// join the 2 arrays, by looping over the longest of 2 arrays
const maxLen = Math.max(argNames.length, argValues.length)
const args = []
for (i = 0; i < maxLen; i++) {
args.push({name: argNames[i], value: argValues[i]})
}
console.log(args)
// implement the actual function here
}
doSomething(1, 2, 3, 4)
and the logged object will be
[
{
"name": "a",
"value": 1
},
{
"name": "c",
"value": 3
},
{
"value": 4
}
]
And here's a working example https://tonicdev.com/5763eb77a945f41300f62a79/5763eb77a945f41300f62a7a
Source: Stackoverflow.com