Question

Is there a syntax for passing composite keys, i.e. lists and objects,
like the below example, or is that by-design?

> obj = {[1, 2]: 3};
SyntaxError: Unexpected token [

The second example works fine, it’s not bad but I’d like to know if there is an alternative way.

> obj = {};
> obj[[1, 2]] = 3;
3
> [1, 2] in obj;
> true

Answer 1

Object property names in JavaScript are at the end just strings, your second example seems to work because the bracket property accessor converts the [1, 2] expression to String (returning "1,2"), for example:

var obj = {};
obj[[1, 2]] = 3;

console.log(obj["1,2"]); // 3

Another example:

var foo = { toString: function () { return "bar"; } },
    obj = {};

obj[foo] = 3; // foo is converted to String ("bar")
console.log(obj["bar"]); // 3

See also:

• jshashtable