What improvements will the Spread Operator implementation bring to javascript?


I'm taking a look at the new features of EcmaScript6 and saw that the Spread Operator been implemented.

It looks very similar to PHP's variadic function (which also uses Spread Operator ).

Here's an example of what the difference between current encodings and Ecmascript6 :


var max = Math.max.apply(null, [1, 2, 3]);


var max = Math.max(...[1, 2, 3]);

And also in the declaration of functions:


 function myFunc(name) {      
    var args = [].slice.call(arguments, 1);
    // faça o restante


  function myFunc(name, ...args)
     // faça o restante

In PHP, the implementation of variadic function brought some improvements as improvements:

  • Not having to use call_user_func_array

  • Not having to use func_get_args .

I would like to know that, in javascript, what will be impactful with this implementation of Spread Operadators ?


The question itself already brings two of these improvements, right? Well, almost. In fact, the second example is quite different from the first.

The first example shows what is often called a spread operator (*): it comes to the left of a list (an Iterable , usually an array) and spreads or "unfolds" its contents into separate variables. In your example, list items are distributed as arguments at function call time:

var max = Math.max(...[1, 2, 3]);  // equivale a Math.max(1, 2, 3)

In ES5, as you've noticed, this is only possible with apply , which is much less readable.

The second example has the opposite semantics: in a function's parameter list, the ... does not distribute a list into values, but rather collects the values ​​in a list:

function myFunc(name, ...args) { } // args coleta todos os argumentos passados depois de name

This is often called rest parameters , or remaining parameters. In the role signature, they can only appear at the end. Regardless of the name, it is clearly a syntax improvement, as it eliminates the need for slice.call(arguments, n) within the function, which would be needed in ES5.

Going back to the spread operator , it has other uses besides what you showed in the first example. In literal arrays, it allows you to concatenate and interpolate arrays:

let umaArray = [3, 4];
let outraArray = [6, 7];
let combo = [1, 2, ...umaArray, 5, ...outraArray];
// combo = [1, 2, 3, 4, 5, 6, 7]

It can also be used on the left side of a destruct operation ( destructuring , very nice feature, a bit like the PHP list , but more powerful):

[a, ...outros] = [1,2,3,4];
// a = 1
// outros = [2, 3, 4]

Finally, it still allows you to do something that required some juggling in ES5, a kind of apply combined with new :

new Date(...[2015,7, 31]);

The ES5 equivalent was this "beauty" here:

new (Date.bind.apply(Date, [null, 2015, 7, 31]));


(*) In the specification this is not even listed as an operator; the language grammar treats the ... literally, and distinguishes the semantics of spread and rest according to context.

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