JS - Create object shorthand

Question

Take the below object:

var value = 'bar';
var obj = { foo: value }
// -> Object { foo="bar" }

Supposing the key was also a variable, one could go:

var key = 'foo', value = 'bar';
var obj = {}
obj[key] = value;
// -> Object { foo="bar" }

Now, I want to do this in one line (shorthand). So I tried:

var obj = {}[key] = value; // or,
var obj = ({})[key] = value; // or,
var obj = new Object()[key] = value;
// -> "bar"

This oddly produces a String instead of an Object.

Is there any way to do this shorthand?

Answer 1

ECMAScript 6 will allow you to do

var obj = {
    [key]: value
};

Browser support is not great yet, but transpilers such as 6to5 allow you to use this notation today!

Answer 2

You can, or almost can. If you put the creation of the object, and its assignment to obj in parentheses, you can set a property of the result of that expression:

var obj, key='foo', value = 'bar';
(obj = {})[key] = value;

alert(obj);  // [object Object]
alert(obj.foo);  // bar

The var keyword cannot be included in that expression within parentheses, so either you will have to declare obj before (like I did), or not declare it at all and accept that it will be in global scope.