How to concat arrays alternatively every 2 items? JS

debugcn Published at Dev

Aerra

I have:

array1 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
array2 = ['a', 'b', 'c'];

I got the concat of arrays working, but only one at a time:

var alternatingArrayResult = [array1, array2].reduce(function (r, a) {
                        return a.forEach(function (a, i) {
                            return (r[i] = r[i] || []).push(a);
                        }), r;
                    }, []).reduce(function (a, b) {
                        return a.concat(b);
                    });

// => alternatingArrayResult = [1, 'a', 2, 'b', 3, 'c', 4, 5, 6, 7, 8, 9];

I want to add two items from array 1, then one from array 2 - and so on.

Desired output:

result = [1, 2, 'a', 3, 4, 'b', 5, 6, 'c', 7, 8, 9];

var array1 = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var array2 = ['a', 'b', 'c'];
var alternatingResultArray = [array1, array2].reduce(function (r, a) {
                    return a.forEach(function (a, i) {
                        return (r[i] = r[i] || []).push(a);
                    }), r;
                }, []).reduce(function (a, b) {
                    return a.concat(b);
                });
                    
console.log(alternatingResultArray);

Nina Scholz

You could get a lenght and iterate the parts for pushing.

const
    array1 = [1, 2, 3, 4, 5, 6, 7, 8, 9],
    array2 = ['a', 'b', 'c'],
    result = [];

for (i = 0, l = Math.max(Math.ceil(array1.length / 2), array2.length); i < l; i++) {
    result.push(
        ...array1.slice(i * 2, (i + 1) * 2),
        ...array2.slice(i, i + 1),
    );
}

console.log(...result);

A more abstact version with size of the wanted subarrays.

const
    array1 = [1, 2, 3, 4, 5, 6, 7, 8, 9],
    array2 = ['a', 'b', 'c'],
    data = [array1, array2],
    sizes = [2, 1]
    result = [];

for (i = 0, l = Math.max(...data.map(({ length }, i) => Math.ceil(length / sizes[i]))); i < l; i++) {
    data.forEach((a, j) => result.push(...a.slice(i * sizes[j], (i + 1) * sizes[j])));
}

console.log(...result);

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