How can i name object "keys" programmatically in JavaScript?

Question

What i want is something i do every day in Python. In JS it does not work, most probably because i suck in JS.

This is the Python code that would do what i want:

>key = 'just_a_key'
>value = 5
>dict = {key: value}
>dict
{'just_a_key':5}
>dict['just_a_key'}
5

This does not work in JS, the key there will be just named 'key'. I know this is because i'm not really creating a dictionary here, but using json to create an object when i write the same code. So my question now is: How can i programmatically name the objects "keys" (or properties?) in JS?

Answer 1

Like this:

dict = {};

dict[key] = value;

JavaScript does not evaluate expressions for property names in its object literal notation, but it does if you use the square bracket form of the object member operator.

MDN Member Operators

Answer 2

Update: this answer was originally written in 2012, but there's a better way now using ES2015/ES6 Computed Property Names. The previous example (see below) could now be written as follows:

const key = 'just_a_key';

const obj = { [key]: 5 };

If you're unfamiliar with object-literal syntax in JavaScript, please note that the important aspect here is the brackets around the dynamic key name (i.e., [key]). The brackets indicate that key (or any other contents) should be interpreted as an expression, rather than being used as a literal value.

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Original answer / pre-ES6 solution:

var obj = {},
    key = 'just_a_key';

obj[key] = 5;