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.