How does Symbol.keyFor() work?

Question

For the below code,

let obj = {
    [Symbol('my_key')]: 1,
    enum: 2,
    nonEnum: 3
}

document.write(Symbol.keyFor(Symbol.for('my_key')) + '<br>'); //  retrieves 'my_key'
document.write(Symbol.keyFor(Object.getOwnPropertySymbols(obj)[0])); //  does not retrieve 'my_key'

below is the output:

my_key
undefined

Why Symbol.keyFor() does not retrieve token string in second case?

Related: [What is 'global symbol registry'?](https://stackoverflow.com/q/31897015/1048572) and [Get the description of a ES6 Symbol](https://stackoverflow.com/q/30301728/1048572) — Bergi, May 12 '18 at 16:04

madox2 · Accepted Answer · 2018-05-12T13:19:18.547

1

From MDN:

The Symbol.keyFor(sym) method retrieves a shared symbol key from the global symbol registry for the given symbol.

In your first case you are using global symbol but as object key using local symbol:

// local symbol
Symbol('my_key')

// global symbol
Symbol.for('my_key')

When you use global symbol in your object literal it will work as youw expect:

let obj = {
    [Symbol.for('my_key')]: 1,
    enum: 2,
    nonEnum: 3
}

Global symbols are stored in global symbol registry. How do you store local symbols is up to you. Here is demonstrated the difference between the two:

Symbol('my_key') === Symbol('my_key') // false
Symbol.for('my_key') === Symbol.for('my_key') // true

edited May 12 '18 at 13:19

answered May 12 '18 at 08:57

madox2

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`[Symbol.for('my_key')]: 1`. Is this creating new symbol in symbol registry instead of local call stack? – overexchange May 12 '18 at 09:27
@overexchange yes, it will create when it does not exist. – madox2 May 12 '18 at 09:30
When you say local symbol, you mean in scope of `window.obj`? – overexchange May 12 '18 at 10:18
@overexchange local symbols are not stored in global symbol registry. You can store it in variable instead – madox2 May 12 '18 at 13:16
Is `Symbol.iterator` a cross-realm symbol? – overexchange May 14 '18 at 14:48

How does Symbol.keyFor() work?

1 Answers1