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

{ 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 literalvar aniArgs = {};// Assign the variable property name with a value of 10aniArgs[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.

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.

ES5 quote that says it should not work

Note: rules have changed for ES6: https://stackoverflow.com/a/2274327/895245

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).