How to use a variable for a key in a JavaScript object literal? [Answered]

Problem explained:

Why does the following work?

<something>.stop().animate(
    { 'top' : 10 }, 10
);

Whereas this doesn’t work:

var thetop = 'top';
<something>.stop().animate(
    { thetop : 10 }, 10
);

To make it even clearer: At the moment I’m not able to pass a CSS property to the animate function as a variable.

How to use a variable for a key in a JavaScript object literal? Answer #1:

{ thetop : 10 } is a valid object literal. The code will create an object with a property named thetop that has a value of 10. Both the following are the same:

obj = { thetop : 10 };
obj = { "thetop" : 10 };

In ES5 and earlier, you cannot use a variable as a property name inside an object literal. Your only option is to do the following:

var thetop = "top";

// create the object literal
var aniArgs = {};

// Assign the variable property name with a value of 10
aniArgs[thetop] = 10; 

// Pass the resulting object to the animate method
<something>.stop().animate(
    aniArgs, 10  
);  

ES6 defines ComputedPropertyName as part of the grammar for object literals, which allows you to write the code like this:

var thetop = "top",
    obj = { [thetop]: 10 };

console.log(obj.top); // -> 10

You can use this new syntax in the latest versions of each mainstream browser.

Answer #2:

With ECMAScript 2015 you are now able to do it directly in object declaration with the brackets notation: 

var obj = {
  [key]: value
}

Where key can be any sort of expression (e.g. a variable) returning a value.

So here your code would look like:

<something>.stop().animate({
  [thetop]: 10
}, 10)

Where thetop will be evaluated before being used as key.

Answer #3:

S5 quote that says it should not work

Spec: http://www.ecma-international.org/ecma-262/5.1/#sec-11.1.5

PropertyName :

• IdentifierName
• StringLiteral
• NumericLiteral

[…]

The production PropertyName : IdentifierName is evaluated as follows:

1. Return the String value containing the same sequence of characters as the IdentifierName.

The production PropertyName : StringLiteral is evaluated as follows:

1. Return the SV [String value] of the StringLiteral.

The production PropertyName : NumericLiteral is evaluated as follows:

1. Let nbr be the result of forming the value of the NumericLiteral.
2. Return ToString(nbr).

This means that:

• { theTop : 10 } is the exact same as { 'theTop' : 10 }The PropertyName theTop is an IdentifierName, so it gets converted to the 'theTop' string value, which is the string value of 'theTop'.
• It is not possible to write object initializers (literals) with variable keys.The only three options are IdentifierName (expands to string literal), StringLiteral, and NumericLiteral (also expands to a string).

Answer #4:

ES6 / 2020

If you’re trying to push data to an object using “key:value” from any other source, you can use something like this:

let obj = {}
let key = "foo"
let value = "bar"

obj[`${key}`] = value

// A `console.log(obj)` would return:
// {foo: "bar}

// A `typeof obj` would return:
// "object"

Hope this helps someone 🙂

Answer #5:

I have used the following to add a property with a “dynamic” name to an object:

var key = 'top';
$('#myElement').animate(
   (function(o) { o[key]=10; return o;})({left: 20, width: 100}),
   10
);

key is the name of the new property.

The object of properties passed to animate will be {left: 20, width: 100, top: 10}

This is just using the required [] notation as recommended by the other answers, but with fewer lines of code!

Answer #6:

I couldn’t find a simple example about the differences between ES6 and ES5, so I made one. Both code samples create exactly the same object. But the ES5 example also works in older browsers (like IE11), wheres the ES6 example doesn’t.

ES6

var matrix = {};
var a = 'one';
var b = 'two';
var c = 'three';
var d = 'four';

matrix[a] = {[b]: {[c]: d}};

ES5

var matrix = {};
var a = 'one';
var b = 'two';
var c = 'three';
var d = 'four';

function addObj(obj, key, value) {
  obj[key] = value;
  return obj;
}

matrix[a] = addObj({}, b, addObj({}, c, d));

Answer #7:

Given code:

var thetop = 'top';
<something>.stop().animate(
    { thetop : 10 }, 10
);

Translation:

var thetop = 'top';
var config = { thetop : 10 }; // config.thetop = 10
<something>.stop().animate(config, 10);

As you can see, the { thetop : 10 } declaration doesn’t make use of the variable thetop. Instead it creates an object with a key named thetop. If you want the key to be the value of the variable thetop, then you will have to use square brackets around thetop:

var thetop = 'top';
var config = { [thetop] : 10 }; // config.top = 10
<something>.stop().animate(config, 10);

The square bracket syntax has been introduced with ES6. In earlier versions of JavaScript, you would have to do the following:

var thetop = 'top';
var config = (
  obj = {},
  obj['' + thetop] = 10,
  obj
); // config.top = 10
<something>.stop().animate(config, 10);

Hope you learned something from this post.

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