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Flatten an Array (ES6)

To flatten an array with depth of level 1:

const inputArray = [1, 2, 3, [4, 5]];
console.log([].concat(...inputArray));
// Output: [1, 2, 3, 4, 5]

Demo

To flatten an array of a specified depth:

const input = [1, 2, 3, [4, 5, 6, [8, 9, 10]], 11, [12, 13, 14]];

const flatten = (arr, depth) =>
  depth === 0 ? arr :
  arr.reduce((acc, curr) =>
    Array.isArray(curr) ? acc.concat(flatten(curr, depth - 1)) :
    acc.concat(curr), [])

console.log(flatten(input, 3));
// Ouput: [1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14]

Demo

To flatten an array infinitely:

const input = [1, 2, 3, [4, 5, 6, [8, 9, 10]], 11, [12, 13, 14]];

const flatten = (arr) =>
  arr.reduce((acc, curr) =>
    Array.isArray(curr) ? acc.concat(flatten(curr)) :
    acc.concat(curr), [])

console.log(flatten(input));
// Ouput: [1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14]

Demo