Using es6 spread to concat multiple arrays

Question

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 dynami

Answer 1

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);