[javascript] How to interpolate variables in strings in JavaScript, without concatenation?

I know in PHP we can do something like this:

$hello = "foo";
$my_string = "I pity the $hello";

Output: "I pity the foo"

I was wondering if this same thing is possible in JavaScript as well. Using variables inside strings without using concatenation — it looks more concise and elegant to write.

Don't see any external libraries mentioned here, but Lodash has _.template(),

https://lodash.com/docs/4.17.10#template

If you're already making use of the library it's worth checking out, and if you're not making use of Lodash you can always cherry pick methods from npm npm install lodash.template so you can cut down overhead.

Simplest form -

var compiled = _.template('hello <%= user %>!');
compiled({ 'user': 'fred' });
// => 'hello fred!'

There are a bunch of configuration options also -

_.templateSettings.interpolate = /{{([\s\S]+?)}}/g;
var compiled = _.template('hello {{ user }}!');
compiled({ 'user': 'mustache' });
// => 'hello mustache!'

I found custom delimiters most interesting.

well you could do this, but it's not esp general

'I pity the $fool'.replace('$fool', 'fool')

You could easily write a function that does this intelligently if you really needed to

You can use this javascript function to do this sort of templating. No need to include an entire library.

function createStringFromTemplate(template, variables) {
    return template.replace(new RegExp("\{([^\{]+)\}", "g"), function(_unused, varName){
        return variables[varName];
    });
}

createStringFromTemplate(
    "I would like to receive email updates from {list_name} {var1} {var2} {var3}.",
    {
        list_name : "this store",
        var1      : "FOO",
        var2      : "BAR",
        var3      : "BAZ"
    }
);

Output: "I would like to receive email updates from this store FOO BAR BAZ."

Using a function as an argument to the String.replace() function was part of the ECMAScript v3 spec. See this SO answer for more details.

Simply use:

var util = require('util');

var value = 15;
var s = util.format("The variable value is: %s", value)

If you're trying to do interpolation for microtemplating, I like Mustache.js for that purpose.

I wrote this npm package stringinject https://www.npmjs.com/package/stringinject which allows you to do the following

var string = stringInject("this is a {0} string for {1}", ["test", "stringInject"]);

which will replace the {0} and {1} with the array items and return the following string

"this is a test string for stringInject"

or you could replace placeholders with object keys and values like so:

var str = stringInject("My username is {username} on {platform}", { username: "tjcafferkey", platform: "GitHub" });

"My username is tjcafferkey on Github" 

var hello = "foo";

var my_string ="I pity the";

console.log(my_string, hello)

Prior to Firefox 34 / Chrome 41 / Safari 9 / Microsoft Edge, nope, that was not possible in javascript. You would have to resort to:

var hello = "foo";
var my_string = "I pity the " + hello;

Peace quote of 2020:

Console.WriteLine("I {0} JavaScript!", ">:D<");

console.log(`I ${'>:D<'} C#`)

String.prototype.interpole = function () {
    var c=0, txt=this;
    while (txt.search(/{var}/g) > 0){
        txt = txt.replace(/{var}/, arguments[c]);
        c++;
    }
    return txt;
}

Uso:

var hello = "foo";
var my_string = "I pity the {var}".interpole(hello);
//resultado "I pity the foo"

Create a method similar to String.format() of Java

StringJoin=(s, r=[])=>{
  r.map((v,i)=>{
    s = s.replace('%'+(i+1),v)
  })
return s
}

use

console.log(StringJoin('I can %1 a %2',['create','method'])) //output: 'I can create a method'

I would use the back-tick ``.

let name1 = 'Geoffrey';
let msg1 = `Hello ${name1}`;
console.log(msg1); // 'Hello Geoffrey'

But if you don't know name1 when you create msg1.

For exemple if msg1 came from an API.

You can use :

let name2 = 'Geoffrey';
let msg2 = 'Hello ${name2}';
console.log(msg2); // 'Hello ${name2}'

const regexp = /\${([^{]+)}/g;
let result = msg2.replace(regexp, function(ignore, key){
    return eval(key);
});
console.log(result); // 'Hello Geoffrey'

It will replace ${name2} with his value.

Complete answer, ready to be used:

 var Strings = {
        create : (function() {
                var regexp = /{([^{]+)}/g;

                return function(str, o) {
                     return str.replace(regexp, function(ignore, key){
                           return (key = o[key]) == null ? '' : key;
                     });
                }
        })()
};

Call as

Strings.create("My firstname is {first}, my last name is {last}", {first:'Neo', last:'Andersson'});

To attach it to String.prototype:

String.prototype.create = function(o) {
           return Strings.create(this, o);
}

Then use as :

"My firstname is ${first}".create({first:'Neo'});

Prior to Firefox 34 / Chrome 41 / Safari 9 / Microsoft Edge, no. Although you could try sprintf for JavaScript to get halfway there:

var hello = "foo";
var my_string = sprintf("I pity the %s", hello);

If you like to write CoffeeScript you could do:

hello = "foo"
my_string = "I pity the #{hello}"

CoffeeScript actually IS javascript, but with a much better syntax.

For an overview of CoffeeScript check this beginner's guide.