Super simple polyfill for Array#flat

array-flatten.js

/** Super simple Polyfill for Array#flat
 * @experimental
 * @interface
 * Supports ie9+, chromeAll, firefox4+, safari5+, opera10.5+
 * Add https://github.com/es-shims/es5-shim to your project to support older browsers
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat
 */

/**
 * flattens array
 * @function flatten
 * @private
 * @param {any} arg - the input to flatten if it is an array
 * @returns {Array} - Flattend array.
 */
function flatten (arg) {
  return arg.reduce(function (accumulator, currentValue) {
    return accumulator.concat(
      Array.isArray(currentValue) ? flatten(currentValue) : currentValue
    )
  }, [])
}

/**
 * @function flat
 * @returns {Array} - flattened array
 */

if (!Array.prototype.hasOwnProperty('flat')) {
  /* eslint-disable no-extend-native */
  Object.defineProperty(Array.prototype, 'flat', {
    value: function () {
      return flatten(this)
    }
  })
  /* eslint-enable no-extend-native */
}

array-flatten.min.js

function flatten(t){return t.reduce(function(t,r){return t.concat(Array.isArray(r)?flatten(r):r)},[])}Array.prototype.hasOwnProperty("flat")||Object.defineProperty(Array.prototype,"flat",{value:function(){return flatten(this)}});

array-flatten.spec.js

/* globals describe,it,expect */
/* To test, run jest */
require('./array-flatten')
describe('Array#flatten', () => {
  it('flattens array', () => {
    let input = [[1, 2, [3]], 4]
    expect(input.flat()).toEqual([1, 2, 3, 4])
  })
})