How to convert instance of any type to string?

I’m implementing a function that receives an argument which it needs to convert to its string representation.

If a given object implements a toString() method, then the function should use it. Otherwise, the function can rely on what the JavaScript implementation offers.

What I come up with is like this:

var convert = function (arg) {
  return (new String(arg)).valueOf();
}

I’m not sure you even need a function, but this would be the shortest way:

function( arg ) {
    return arg + '';
}

Otherwise this is the shortest way:

arg += '';

String(null) returns – “null”

String(undefined) returns – “undefined”

String(10) returns – “10”

String(1.3) returns – “1.3”

String(true) returns – “true”

I think this is a more elegent way.

value = value+"";

All data types in JavaScript inherit a toString method:

('hello').toString();   // "hello"
(123).toString();       // "123"
([1,2,3]).toString();   // "1,2,3"
({a:1,b:2}).toString(); // "[object Object]"
(true).toString();      // "true"

If targeting ES6 or later, you could use a template literal:

function (arg) {
    return `${arg}`;
}

The other answers are incomplete when it comes to a JSON object passed. So I made this one and it works for all:

var getString = (o) => {
    if (o !== null) {
        if (typeof o === 'string') {
            return o;
        } else {
            return JSON.stringify(o);
        }
    } else {
        return null;
    }
}

JSON.stringify(value)

Works for null, undefined, primitives, arrays and objects — basically everything.