ES6 - generate an array of numbers

Having googled for it I found two solutions:

    var data = [...Array(10).keys()];
console.log(data);
    var data1 = Array(8).fill().map((_, i) => i);
console.log(data1);

data1 displays [0, 1, ..., 7] however data just displays [[object Array Iterator]] how do I actually see the numbers.

I need it for some iterations over numbers (part of Euler project).

Previously I did a lot of Euler challenges in Python. Now I decided I'll revisit it and do as much as I can in JS (as much ES6 syntax as possible) to help me develop my js skills.

Here is a simple solution that works in codepen:

Array.from(Array(10).keys())

To be clear, Array.from() and Array.keys() require an ES6 polyfill in order to work in all browsers.

A tour of Array.from thru practical examples

Array.from also accepts a second argument which is used as a mapping function

let out = Array.from(Array(10), (_,x) => x);
console.log(out);
// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

This is nice to know because you might want to generate arrays that are sometimes more complex than just 0 thru N.

const sq = x => x * x;
let out = Array.from(Array(10), (_,x) => sq(x));
console.log(out);
// [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Or you can make arrays out of generators, too

function* range(start, end, step) {
  while (start < end) {
    yield start;
    start += step;
  }
}

let out = Array.from(range(10,20,2));

console.log(out); // [10, 12, 14, 16, 18]

Array.from is just massively powerful. People don't even realize its full potential yet.

const ord = x => x.charCodeAt(0);
const dec2hex = x => `0${x.toString(16)}`.substr(-2);

// common noob code
{
  let input = "hello world";
  let output = input.split('').map(x => dec2hex(ord(x)));
  
  console.log(output);
  // ["68", "65", "6c", "6c", "6f", "20", "77", "6f", "72", "6c", "64"]
}

// Array.from
{
  let input = "hello world";
  let output = Array.from(input, x => dec2hex(ord(x)));
  
  console.log(output);
  // ["68", "65", "6c", "6c", "6f", "20", "77", "6f", "72", "6c", "64"]
}

It seems the problem is that codepen precompiles your code using babel es2015-loose.

In that mode, your

[...Array(10).keys()];

becomes

[].concat(Array(10).keys());

And that's why you see an array containing an iterator.

With es2015 mode you would get

function _toConsumableArray(arr) {
  if (Array.isArray(arr)) {
    for (var i = 0, arr2 = Array(arr.length); i < arr.length; i++) {
      arr2[i] = arr[i];
    }
    return arr2;
  } else {
    return Array.from(arr);
  }
}
[].concat(_toConsumableArray(Array(10).keys()));

which would behave as desired.

See ②ality - Babel 6: loose mode for more information about the modes.

All of the other answers here create a temporary intermediate array, which isn't necessary.

Array.from({ length: 10 }, (_, i) => i)

This is essentially a map function where you map from array index to whatever you'd like, for any number of elements.

Further refinement, produces an array with value starting from 1:

Array.from(Array(10).keys(), n => n + 1)

Here is a range function which takes start, end, and a step parameter. It returns an array starting a from start upto (but excluding) the end number with increments of size step.

const range = (start, end, step) => { return Array.from(Array.from(Array(Math.ceil((end-start)/step)).keys()), x => start+ x*step); }

console.log(range(1, 10, 1)); //[1, 2, 3, 4, 5, 6, 7, 8, 9]

console.log(range(0, 9, 3)); //[0, 3, 6]

console.log(range(10, 30, 5)); //[10, 15, 20, 25]

Taking a step further, if you want a range that includes the end as well.

const inclusiveRange = (start, end, step) => { return Array.from(Array.from(Array(Math.ceil((end-start+1)/step)).keys()), x => start+ x*step); }

console.log(inclusiveRange(1, 10, 1)); //[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

console.log(inclusiveRange(0, 9, 3)); // [0, 3, 6, 9]

console.log(inclusiveRange(10, 30, 5)); //[10, 15, 20, 25, 30]