In PHP you can do amazing/horrendous things like this:
$a = 1;
$b = 2;
$c = 3;
$name = 'a';
echo $$name;
// prints 1
Is there any way of doing something like this with Javascript?
E.g. if I have a var name = 'the name of the variable';
can I get a reference to the variable with name name
?
This question is related to
javascript
dynamic-variables
Just don't know what a bad answer gets so many votes. It's quite easy answer but you make it complex.
// If you want to get article_count
// var article_count = 1000;
var type = 'article';
this[type+'_count'] = 1000; // in a function we use "this";
alert(article_count);
This is an example :
for(var i=0; i<=3; i++) {
window['p'+i] = "hello " + i;
}
alert(p0); // hello 0
alert(p1); // hello 1
alert(p2); // hello 2
alert(p3); // hello 3
Another example :
var myVariable = 'coco';
window[myVariable] = 'riko';
alert(coco); // display : riko
So, the value "coco" of myVariable becomes a variable coco.
Because all the variables in the global scope are properties of the Window object.
It is always better to use create a namespace and declare a variable in it instead of adding it to the global object. We can also create a function to get and set the value
See the below code snippet:
//creating a namespace in which all the variables will be defined.
var myObjects={};
//function that will set the name property in the myObjects namespace
function setName(val){
myObjects.Name=val;
}
//function that will return the name property in the myObjects namespace
function getName(){
return myObjects.Name;
}
//now we can use it like:
setName("kevin");
var x = getName();
var y = x;
console.log(y) //"kevin"
var z = "y";
console.log(z); //"y"
console.log(eval(z)); //"kevin"
In this similar way, we can declare and use multiple variables. Although this will increase the line of code but the code will be more robust and less error-prone.
eval() did not work in my tests. But adding new JavaScript code to the DOM tree is possible. So here is a function that adds a new variable:
function createVariable(varName,varContent)
{
var scriptStr = "var "+varName+"= \""+varContent+"\""
var node_scriptCode = document.createTextNode( scriptStr )
var node_script = document.createElement("script");
node_script.type = "text/javascript"
node_script.appendChild(node_scriptCode);
var node_head = document.getElementsByTagName("head")[0]
node_head.appendChild(node_script);
}
createVariable("dynamicVar", "some content")
console.log(dynamicVar)
If you don't want to use a global object like window or global (node), you can try something like this:
var obj = {};
obj['whatever'] = 'There\'s no need to store even more stuff in a global object.';
console.log(obj['whatever']);
This is an alternative for those who need to export a dynamically named variable
export {
[someVariable]: 'some value',
[anotherVariable]: 'another value',
}
// then.... import from another file like this:
import * as vars from './some-file'
Another alternative is to simply create an object whose keys are named dynamically
const vars = { [someVariable]: 1, [otherVariable]: 2 };
// consume it like this
vars[someVariable];
Although this have an accepted answer I would like to add an observation:
In ES6 using let
doesn't work:
/*this is NOT working*/_x000D_
let t = "skyBlue",_x000D_
m = "gold",_x000D_
b = "tomato";_x000D_
_x000D_
let color = window["b"];_x000D_
console.log(color);
_x000D_
However using var
works
/*this IS working*/_x000D_
var t = "skyBlue",_x000D_
m = "gold",_x000D_
b = "tomato";_x000D_
_x000D_
let color = window["b"];_x000D_
console.log(color);
_x000D_
I hope this may be useful to some.
eval
is one option.
var a = 1;
var name = 'a';
document.write(eval(name)); // 1
In Javascript you can use the fact that all properties are key value pairs. jAndy already mentioned this but I don't think his answer show how it can be exploited.
Usually you are not trying to create a variable to hold a variable name but are trying to generate variable names and then use them. PHP does it with $$var
notation but Javascript doesn't need to because property keys are interchangeable with array keys.
var id = "abc";
var mine = {};
mine[id] = 123;
console.log(mine.abc);
gives 123. Usually you want to construct the variable which is why there is the indirection so you can also do it the other way around.
var mine = {};
mine.abc = 123;
console.log(mine["a"+"bc"]);
This will do exactly what you done in php:
var a = 1;
var b = 2;
var ccc = 3;
var name = 'a';
console.log( window[name] ); // 1
what they mean is no, you can't. there is no way to get it done. so it was possible you could do something like this
function create(obj, const){
// where obj is an object and const is a variable name
function const () {}
const.prototype.myProperty = property_value;
// .. more prototype
return new const();
}
having a create function just like the one implemented in ECMAScript 5.
I needed to draw multiple FormData on the fly and object way worked well
var forms = {}
Then in my loops whereever i needed to create a form data i used
forms["formdata"+counter]=new FormData();
forms["formdata"+counter].append(var_name, var_value);
use Object is great too.
var a=123
var b=234
var temp = {"a":a,"b":b}
console.log(temp["a"],temp["b"]);
You can use the window object to get at it .
window['myVar']
window
has a reference to all global variables and global functions you are using.
a = 'varname';
str = a+' = '+'123';
eval(str)
alert(varname);
Try this...
2019
eval
operator can run string expression in the context it called and return variables from that context; literal object
theoretically can do that by write:{[varName]}
, but it blocked by definition. So I come across this question and everyone here just play around without bringing a real solution. but @Axel Heider has a good approaching.
The solution is eval
.
almost most forgotten operator. ( think most one is with()
)
eval
operator can dynamically run expression in the context it called. and return the result of that expression. we can use that to dynamically return a variable's value in function's context.
example:
function exmaple1(){
var a = 1, b = 2, default = 3;
var name = 'a';
return eval(name)
}
example1() // return 1
function example2(option){
var a = 1, b = 2, defaultValue = 3;
switch(option){
case 'a': name = 'a'; break;
case 'b': name = 'b'; break;
default: name = 'defaultValue';
}
return eval (name);
}
example2('a') // return 1
example2('b') // return 2
example2() // return 3
Note that I always write explicitly the expression eval
will run.
To avoid unnecessary surprises in the code. eval
is very strong
But I'm sure you know that already
BTW, if it was legal we could use literal object
to capture the variable name and value, but we can’t combine computed property names and property value shorthand, sadly, is invalid
functopn example( varName ){
var var1 = 'foo', var2 ='bar'
var capture = {[varName]}
}
example('var1') //trow 'Uncaught SyntaxError: Unexpected token }`
Source: Stackoverflow.com