We all know you can do:
let arr1 = [1,2,3];
let arr2 = [3,4,5];
let arr3 = [...arr1, ...arr2]; // [1,2,3,3,4,5]
But how do you make this dynamic to concat N arrays?
10 Answers
One option is to use reduce:
let arrs = [[1, 2], [3, 4], [5, 6]];
arrs.reduce((a, b) => [...a, ...b], []);
Of course, this is a slow solution (quadratic time). Alternatively, if you can use Lodash, _.flatten does exactly what you want, and does it more efficiently (linear time).
EDIT
Or, adapted from Xotic750's comment below,
[].concat(...arrs);
Which should be efficient (linear time).
4 Comments
Xotic750
One you have your arrays in an array, you could then.
Array.prototype.concat.apply([], arr) Though this isn't using the spread operator any more.
Brian McCutchon
@Xotic750 Nice, but you can do that using the spread operator! See my edit.
Simon_Weaver
Note that in typescript you need to force the empty array to be typed if you want the return value to be
number[] and not any[] (eg. ([] as number[]).concat(...arrs))
Sebastian
Are there any new other solutions for this?
Another option could be:
const nArrays = [
[1, 2, 3, 4, 5],
[6, 7, 8, 9],
[10, 11]
];
const flattened = [].concat(...nArrays);
console.log(flattened)
1 Comment
cayhorstmann
As of ES2019, you can just use
nArrays.flat()let fruits = ["apples", "bananas", "pears"];
let vegetables = ["corn", "potatoes", "carrots"];
let produce = [...fruits, ...vegetables];
console.log(produce);
1 Comment
yue you
This given code is exactly same as the given question. It is not an answer for making dynamiclly concat solution for N arrays.
Best option is to use FlatMap, which helps us to conact multiple arrays into one single array.
Example:
let arrs = [[1, 2], [3, 4], [5, 6]];
arrs.flatMap(a => a);
result will be
> (6) [1, 2, 3, 4, 5, 6]
Happy Coding...
Comments
Following solution works for me (spread operator in ES6):
let array = ['my','solution','works'];
let newArray = [];
let newArray2 = [];
newArray.push(...array); //adding to same array
newArray2.push([...array]); //adding as child/leaf/sub-array
console.log(newArray);
console.log(newArray2);
Comments
You can't do that with spread syntax alone, as spread syntax requires you to know how many arrays you are concatenating in advance. However, you could write the following function:
function concatN(...arguments) {
let accumulator = [];
for(let arg = 0; arg < arguments.length; arg = arg + 1) {
accumulator = [...accumulator, ...arguments[arg]];
}
return accumulator;
}
It probably won't be very efficient, though (repeated use of spread syntax is O(n²)). Using Array.prototype.concatwould be better. You can just do:
[].concat(all, of, your, arrays);
Comments
You can use spread element within for..of loop to concatenate array values to a single array
let arr1 = [1,2,3];
let arr2 = [3,4,5];
let arr3 = [];
for (let arr of [arr1, arr2 /* , arrN */]) arr3.push(...arr);
console.log(arr3);
Comments
You could use a recursive function and Array.prototype.concat
const concatN = (x,...xs) =>
x === undefined ? [] : x.concat(concatN(...xs))
console.log(concatN([1,2,3], [4,5,6], [7,8,9]))
// [1,2,3,4,5,6,7,8,9]You can do the same thing using reduce and Array.prototype.concat. This is similar to the accepted answer but doesn't senselessly use spread syntax where x.concat(y) is perfectly acceptable (and likely heaps faster) in this case
const concatN = (...xs) =>
xs.reduce((x,y) => x.concat(y), [])
console.log(concatN([1,2,3], [4,5,6], [7,8,9]))
// [1,2,3,4,5,6,7,8,9]
Comments
We can resolve using es6 following way
function mergeTwo(arr1, arr2) {
let result = [...arr1, ...arr2];
return result.sort((a,b) => a-b);
}
Comments
let arr1 = [1,2,3];
let arr2 = [3,4,5];
let arrs = [arr1, arr2].flat(); // [1,2,3,3,4,5]
console.log(arrs);
concatmethod still works...Function.prototype.call.bind(Array.prototype.concat):)