Array.prototype.flatten = Array.prototype.flatten || function() {
    return [].reduce.call(this, function(flat, toFlatten) {
        return flat.concat(Array.isArray(toFlatten) ? toFlatten.flatten() : toFlatten);
    },[])
};

function flatten(input) {
  let result = [];
  
  function extractArrayElements(input) {
    for(let i = 0; i < input.length; i++){
      if(Array.isArray(input[i])){
        extractArrayElements(input[i]);
      }else{
        result.push(input[i]);
      }
    }
  }
  
  extractArrayElements(input);
  
  return result;
}


// let input = [1,2,3,[4,5,[44,7,8,9]]];
// console.log(flatten(input));

// output [1,2,3,4,5,6,7,8,9]

以下代码将压平深度嵌套的数组：

/**
 * [Function to flatten deeply nested array]
 * @param  {[type]} arr          [The array to be flattened]
 * @param  {[type]} flattenedArr [The flattened array]
 * @return {[type]}              [The flattened array]
 */
function flattenDeepArray(arr, flattenedArr) {
  let length = arr.length;

  for(let i = 0; i < length; i++) {
    if(Array.isArray(arr[i])) {
      flattenDeepArray(arr[i], flattenedArr);
    } else {
      flattenedArr.push(arr[i]);
    }
  }

  return flattenedArr;
}

let arr = [1, 2, [3, 4, 5], [6, 7]];

console.log(arr, '=>', flattenDeepArray(arr, [])); // [ 1, 2, [ 3, 4, 5 ], [ 6, 7 ] ] '=>' [ 1, 2, 3, 4, 5, 6, 7 ]

arr = [1, 2, [3, 4], [5, 6, [7, 8, [9, 10]]]];

console.log(arr, '=>', flattenDeepArray(arr, [])); // [ 1, 2, [ 3, 4 ], [ 5, 6, [ 7, 8, [Object] ] ] ] '=>' [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

只是为了增加伟大的解决方案。我用递归来解决这个问题。

            const flattenArray = () => {
                let result = [];
                return function flatten(arr) {
                    for (let i = 0; i < arr.length; i++) {
                        if (!Array.isArray(arr[i])) {
                            result.push(arr[i]);
                        } else {
                            flatten(arr[i])
                        }
                    }
                    return result;
                }
            }

测试结果：https://codepen.io/ashermike/pen/mKZrWK

我已经使用递归和闭包完成了

function flatten(arr) {

  var temp = [];

  function recursiveFlatten(arr) { 
    for(var i = 0; i < arr.length; i++) {
      if(Array.isArray(arr[i])) {
        recursiveFlatten(arr[i]);
      } else {
        temp.push(arr[i]);
      }
    }
  }
  recursiveFlatten(arr);
  return temp;
}

ES6的纯粹魔力

const flat = A => A.reduce((A, a) => Array.isArray(a) ? [...A, ...flat(a)] : [...A, a], []);