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Ilya Kantor 2017-06-11 16:24:04 +03:00
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1-js/04-object-basics/03-symbol/article.md
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@ -159,14 +159,14 @@ alert( clone[id] ); // 123
There's no paradox here. That's by design. The idea is that when we clone an object or merge objects, we usually want *all* properties to be copied (including symbols like `id`).
````smart header="Property keys of other types are coerced to strings"
We can only use strings or symbols as keys in objects. Other types are coerced to strings.
We can only use strings or symbols as keys in objects. Other types are converted to strings.
For instance:
For instance, a number `0` becomes a string `"0"` when used as a property key:
```js run
let obj = {
0: "test" // same as "0": "test"
}
};
// both alerts access the same property (the number 0 is converted to string "0")
alert( obj["0"] ); // test
@ -176,11 +176,11 @@ alert( obj[0] ); // test (same property)
## Global symbols
Normally, all symbols are different. But sometimes we want same-named symbols to be the same.
As we've seen, usually all symbols are different, even if they have the same name. But sometimes we want same-named symbols to be same entities.
For instance, different parts of our application want to access symbol `"id"` meaning exactly the same property.
To achieve that, there exists a *global symbol registry*. We can create symbols in it and and access them later, and it guarantees that repeated accesses by the same name return exactly the same symbol.
To achieve that, there exists a *global symbol registry*. We can create symbols in it and access them later, and it guarantees that repeated accesses by the same name return exactly the same symbol.
In order to create or read a symbol in the registry, use `Symbol.for(name)`.
@ -230,7 +230,9 @@ alert( Symbol.keyFor(Symbol.for("name")) ); // name, global symbol
alert( Symbol.keyFor(Symbol("name2")) ); // undefined, non-global symbol
```
For non-global symbols, the name is only used for debugging purposes.
So, for global symbols the name may be indeed helpful, as we can get a symbol by id.
And for non-global symbols the name is only used for debugging purposes, like printing out a symbol.
## System symbols