What is a Symbol for in JavaScript? (ES6)

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What is a Symbol for in JavaScript? (ES6)

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#1 by (7 votes)
#2 by (5 votes)

11

Reading the MDN documentation where we talk about the primitive data Symbol for more that I read and reread I do not understand what would be a use case for that element more than to iterate Symbol.iterator .

The documentation says that its use is for debug purposes, but even with this I do not understand why to use Symbol to debug if there is console.log() or debugger .

I understand that it is a primitive data and, therefore, immutable. In the examples I have seen that they do something like the following:

var sym = Symbol('Llave'),
    obj = {
        [sym]: 'valor'
    };

console.log(obj[sym]); // valor
console.log(obj['Llave']); // undefined

Why use a Symbol ? if you can use a traditional key (key: value).

Sources:

javascript ecmascript-2015

asked by Chofoteddy 29.12.2015 в 18:58

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2 answers

5

Since @WilfredoP has not mentioned the global symbols I will take advantage of:

var s1 = Symbol.for('foo');
var s2 = Symbol.for('foo');
s1 === s2 // retorna true, ya que Symbol.for(<name>) genera símbolos globales.

Here, s1 and s2 are basically the same symbol. But how a symbol elbows is not as important as its purpose:

The Symbols are part of a movement that is doing Javascript towards the metaprogramacion . And they are not alone in this, ECMAScript 2015 includes other 2 additions that point in that line that are Proxy and Reflect .

Therefore, its use is very varied. The question is why semantics will have these for your application.

There are some well-known symbols!

Symbol.hasInstance: instanceof

Symbol.hasInstance is the symbol that controls the behavior of instanceof . That is, A instanceof B is equivalent B[Symbol.hasInstance](A) .

Example:

class MiArray {  
    // aquí sobrecargo el operador instanceof 
    static [Symbol.hasInstance](otro) {
        return Array.isArray(otro);
    }
}
assert([] instanceof MiArray); // pasa, por evaluarse como true

Symbol.iterator

Supercharging the objeto[Symbol.iterator] method allows changing the behavior of the of operator in a for .. of block if it is overloaded as a generating function.

Example:

class IterarSaltando {  
  *[Symbol.iterator]() {
    var i = 0;
    while(this[i] !== undefined) {
      yield this[i];
      i+=2;
    }
  }
}
var iterarSaltando = new IterarSaltando();
iterarSaltando[0] = '1';
iterarSaltando[1] = '2';
iterarSaltando[2] = '3';
for(var value of iterarSaltando) {  
    console.log(value); // 1, luego 3
}

There is a fairly long list of these pre-symbols defined by the standard. But not all are 100% implemented in all browsers.

answered by 29.12.2015 в 20:00

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score 7 · Accepted Answer

Symbols are a unique data type that is immutable and can be used as an object property identifier. These are like the types Number , String , and Boolean primitives.

To create it is done in the following way, without using the word new, since the Symbols have a function Symbol which is used to create them;

var sym1 = Symbol();
var sym2 = Symbol("foo");
var sym3 = Symbol("foo");

As mentioned above, being unique types, he will create a new Symbol which will not be another:

Symbol("foo") === Symbol("foo"); // false

About debuging These are considered powerful, since they have a description, which is used only to debug to make life easier when debugging by console:

console.log(Symbol('foo')); // prints "Symbol(foo)" to the console.  
assert(Symbol('foo').toString() === 'Symbol(foo)');

What are they good for?:

To create and store values such as integer and string that will not change.

Can be used to save custom metadata of the objects, which are secondary to the current object.

Conclusion:

They are small constants that have some extra properties, which allow us to work better in debug and save unique values.

A good resource is this page where there is an example, and they explain much more extensive what which are the symbols.