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

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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.

First answer

{ 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.

Second answer

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).

Third answer

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!

Reprint:https://stackoverflow.com/questions/2274242/how-to-use-a-variable-for-a-key-in-a-javascript-object-literal
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