Using Array.map with new Array constructor

问题

I was trying to use new Array() constructor with map in order to create a one-line code that creates a list of elements. Something like this :

let arr = new Array(12).map( (el, i) => {
  console.log('This is never called');
  return i + 1;
});

Reading docs, the behaviour makes sense.

Basically docs say that callback of map will be executed even for declared undefined values in array, but not for example when creating empty Arrays like the code before.

So this should work :

var arr = new Array(12);

for(let i = 0; i < arr.length ; i++){
  arr[i] = undefined;
}

let list = arr.map( (e, i) => {
  console.log(i + 1);
  return i + 1;
});

So, We also can do something like this :

let newArray = (length) => {
  let myArray = new Array(length);
  for(let i = 0; i < length; i++) myArray[i] = undefined;
  return myArray;
};

console.log( newArray(12).map( (el, i) => i + 1 ) );

So my question. Is there a better/pretty way to do it with map function ?

Thanks in advance!

回答1:

You can use Array#from to create an array having passed length.

Array.from({length: 12}, (e, i) => i + 1);

回答2:

Section 22.1.3.15 of the spec is getting you, with the last line of NOTE 1:

callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

This behavior is shared by filter, forEach, and the other functional methods.

The array constructor you are calling, Array(len) from section 22.1.1.2, will set the length of the array but not initialize the elements. I'm not sure where the spec defines what a missing element is, but the array produced by new Array(len) should fit that and so none of the elements will be touched by map.

回答3:

I like this approach because it still works in ES5:

let arr = Array.apply(null, Array(12))
    .map(function (x, i) {
        return i
    });

// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Since New Array returns an array that contains "holes", we use apply because apply treats holes as if they were undefined. This spreads undefined across the entire array which makes it mappable.

回答4:

You could use Array.apply with an object with the length in it.

var array = Array.apply(null, { length: 12 }).map((_, i) => i + 1);
console.log(array);

回答5:

If I am not mistaken, you explicitly said that you wanted an answer with new Array constructor.

Here is how to do it. One line, beautiful ES6:

let me = [...new Array(5)].map((x,i)=>i)
// [0,1,2,3,4]

Explanation:

new Array(5) generates this: [ , , , , ]
[...new Array(5)] is a shorthand for Array.from(new Array(5)), and generates this: [ undefined, undefined, undefined, undefined, undefined ], that is an array of undefined objects. Yes, undefined is a value (a primitive one :) ).
Now we can iterate over the values of our brand new array :D

PS: you can even skip the new keyword and do:

[...Array(5)].map((x,i)=>i)

来源：https://stackoverflow.com/questions/37487602/using-array-map-with-new-array-constructor

标签

javascript

arrays

map-function