Syntax for creating objects with composite keys in JavaScript
Is there a syntax for passing composite keys, i.e. lists and objects,
like the below example开发者_Python百科, 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
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
If you don't want to do string concatenation you can use nested maps, then a wrapper to make the code a less verbose. Here's an example in TypeScript.
class MapMap<Ka, Kb, V> implements Iterable<[Ka, Kb, V]> {
readonly mm = new Map<Ka, Map<Kb, V>>()
get(a: Ka, b: Kb): V | undefined {
const m = this.mm.get(a)
if (m !== undefined) {
return m.get(b)
}
return undefined
}
set(a: Ka, b: Kb, v: V): void {
let m = this.mm.get(a)
if (m === undefined) {
this.mm.set(a, (m = new Map()))
}
m.set(b, v)
}
*[Symbol.iterator](): Iterator<[Ka, Kb, V]> {
for (const [a, m] of this.mm) {
for (const [b, v] of m) {
yield [a, b, v]
}
}
}
}
do you need the [1, 2]
to be preserved as an array? what would this exactly enable you to do? I'm not familiar with "composite keys" so maybe a short explanation for link to one to clarify would help me understand your problem better.
if you just want to use [1, 2]
as a key, you can always use that as a string:
var obj = {
"[1, 2]": 3
}
but again, i would assume you would want to keep [1, 2]
as an array.
精彩评论