Function that recursively flattens an array in vanilla JS with tests

recursively-flatten.js

function recursivelyFlatten(array) {
  if (!array) {
    return []
  }
  
  var result = [];
  
  if (Array.isArray(array)) {
  	array.forEach((value) => {
    	result = result.concat(recursivelyFlatten(value));
    });
  } else {
  	result = [array];
  }
  
  return result
}

function runTest(input, output, message) {
  var expected = JSON.stringify(output);
  var actual = JSON.stringify(recursivelyFlatten(input));
  if ( actual !== expected) {
    throw message + ' expected: ' + expected + ', got: ' + actual;
  }
  
  console.log(message, '✔');
}

const testSuit = {
  withEmptyArray: () => runTest([], [], 'should return empty array if array is empty'),
  zeroLevelsDeep: () => runTest([1,2,3], [1,2,3], 'should flatten array zero levels deep'),
  multipleLevelsDeep: () => runTest([[1,2,[3]],4], [1,2,3,4], 'should flatten array multple levels deep'),
  ignoreEmptyArrays: () => runTest([[], [1,2], [3, []], 4], [1,2,3,4], 'should ignore empty arrays')
}

Object.keys(testSuit).forEach((test) => testSuit[test]());