Merge two arrays with alternating values

Here's another way you can do it using destructuring assignment -

const interleave = ([ x, ...xs ], ys = []) =>
  x === undefined
    ? ys                             // base: no x
    : [ x, ...interleave (ys, xs) ]  // inductive: some x
        
console.log (interleave ([0, 2, 4, 6], [1, 3, 5])) // [ 0 1 2 3 4 5 6 ]    
console.log (interleave ([0, 2, 4], [1, 3, 5, 7])) // [ 0 1 2 3 4 5 7 ]
console.log (interleave ([0, 2, 4], []))           // [ 0 2 4 ]
console.log (interleave ([], [1, 3, 5, 7]))        // [ 1 3 5 7 ]
console.log (interleave ([], []))                  // [ ]

And another variation that supports any number of input arrays -

const interleave = ([ x, ...xs ], ...rest) =>
  x === undefined
    ? rest.length === 0
      ? []                               // base: no x, no rest
      : interleave (...rest)             // inductive: no x, some rest
    : [ x, ...interleave(...rest, xs) ]  // inductive: some x, some rest

console.log (interleave ([0, 2, 4, 6], [1, 3, 5])) // [ 0 1 2 3 4 5 6 ]    
console.log (interleave ([0, 2, 4], [1, 3, 5, 7])) // [ 0 1 2 3 4 5 7 ]
console.log (interleave ([0, 2, 4], []))           // [ 0 2 4 ]
console.log (interleave ([], [1, 3, 5, 7]))        // [ 1 3 5 7 ]
console.log (interleave ([], []))                  // [ ]

You could iterate the min length of both array and build alternate elements and at the end push the rest.

var array1 = ["a", "b", "c", "d"],
    array2 = [1, 2],
    result = [],
    i, l = Math.min(array1.length, array2.length);
    
for (i = 0; i < l; i++) {
    result.push(array1[i], array2[i]);
}
result.push(...array1.slice(l), ...array2.slice(l));

console.log(result);

Solution for an arbitrary count of arrays with a transposing algorithm and later flattening.

var array1 = ["a", "b", "c", "d"],
    array2 = [1, 2],
    result = [array1, array2]
        .reduce((r, a) => (a.forEach((a, i) => (r[i] = r[i] || []).push(a)), r), [])
        .reduce((a, b) => a.concat(b));
    
console.log(result);

Create an array of tuples. Each tuple contains 1 element from each array, flatten by spreading the array of tuples, and adding the leftover items from the arrays:

const a1 = ["a", "b", "c", "d"];
const a2 = [1,2];
const l = Math.min(a1.length, a2.length);

const merged = [].concat(...Array.from({ length: l }, (_, i) => [a1[i], a2[i]]), a1.slice(l), a2.slice(l));
  
console.log(merged);

Here's a modern solution that takes any number of arrays:

const braidArrays = (...arrays) => {
  const braided = [];
  for (let i = 0; i < Math.max(...arrays.map(a => a.length)); i++) {
    arrays.forEach((array) => {
      if (array[i] !== undefined) braided.push(array[i]);
    });
  }
  return braided;
};

Note that you could change Math.max to Math.min to only include up to the shortest array.

Here's a sample I/O:

braidArrays(['a','b','c','d'], [1,2,3], [99,98,97,96,95]);
// ['a', 1, 99, 'b', 2, 98, 'c', 3, 97, 'd', 96, 95]

Merge two arrays with alternating values

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