jeena/en.javascript.info
1
0
Fork 0
Code Issues Pull requests Projects Packages Wiki Activity Actions

symbols

Browse source
This commit is contained in:
Ilya Kantor 2017-06-20 14:46:56 +03:00
parent 138fb2e022
commit 2a0516d174
2 changed files with 31 additions and 23 deletions
Download patch file Download diff file Expand all files Collapse all files

20
1-js/04-object-basics/03-symbol/article.md
View file

@ -252,11 +252,19 @@ Other symbols will also become familiar when we study the corresponding language
## Summary
- Symbol is a primitive type for unique identifiers.
- Symbols are created with `Symbol(name)` call.
- Symbols are useful if we want to create a field that only those who know the symbol can access.
- Symbols don't appear in `for..in` loops.
- Symbols created with `Symbol(name)` are always different, even if they have the same name. If we want same-named symbols to be equal, then we should use the global registry: `Symbol.for(name)` returns (creates if needed) a global symbol with the given name. Multiple calls return the same symbol.
- There are system symbols used by JavaScript and accessible as `Symbol.*`. We can use them to alter some built-in behaviors.
`Symbol` is a primitive type for unique identifiers.
Symbols are created with `Symbol(name)` call.
Symbols are always different values, even if they have the same name. If we want same-named symbols to be equal, then we should use the global registry: `Symbol.for(name)` returns (creates if needed) a global symbol with the given name. Multiple calls of `Symfol.for` return exactly the same symbol.
Symbols have two main use cases:
1. "Hidden" object properties.
If we want to add a property into an object that "belongs" to another script or a library, we can create a symbol and use it as a property key. A symbolic property does not appear in `for..in`, so it won't be occasionally listed. Also it won't be accessed directly, because another script does not have our symbol, so it will not occasionally intervene into its actions.
So we can "covertly" hide something into objects that we need, but others should not see, using symbolic properties.
2. There are many system symbols used by JavaScript and accessible as `Symbol.*`. We can use them to alter some built-in behaviors. For instance, later in the tutorial we'll use `Symbol.iterator` for [iterables](info:iterable), `Symbol.toPrimitive` to setup [object-to-primitive conversion](info:object-toprimitive) and so on.
Technically, symbols are not 100% hidden. There is a build-in method [Object.getOwnPropertySymbols(obj)](mdn:js/Object/getOwnPropertySymbols) that allows to get all symbols. Also there is a method named [Reflect.ownKeys(obj)](mdn:js/Reflect/ownKeys) that returns *all* keys of an object including symbolic ones. So they are not really hidden. But most libraries, built-in methods and syntax constructs adhere to a common agreement that they are. And the one who explicitly calls the aforementioned methods probably understands well what he's doing.