在JavaScript中展平多级/嵌套数组

Question

我正在尝试了解此代码段如何使多级/嵌套数组扁平化

有人可以帮忙详细说明吗？谢谢

var flatten = function(a, shallow,r){
  if(!r){ r = []}     //what does the exclamation mark mean here?...

if (shallow) {
  return r.concat.apply(r,a);  //I can't find what's .apply for concat method
  }

   for(var i=0; i<a.length; i++){
        if(a[i].constructor == Array){
            flatten(a[i],shallow,r);
        }else{
            r.push(a[i]);
        }
    }
    return r;
}

alert(flatten([1, [2], [3, [[4]]],[5,6]]));

Answer 1

有趣的是，在示例中未达到代码的较浅部分。

更新

我认为该功能很难遵循。这是一个可能更容易阅读的替代方法。

function flatten(arr) {

  var flatArray = [];

  function pushLoop(a) {
    var len = a.length;
    var i=0;
    for (i; i < len; i++) {
      if (a[i] && a[i].constructor == Array) {
        pushLoop(a[i]);
      } else {
        flatArray.push(a[i]);
      }
    }
  }

  pushLoop(arr);
  return flatArray;
}

原始片段

function flatten(arr) {

  var flatArray = [];

  function pushLoop(a) {
    var len = a.length;
    var i=0;
    for (i; i < len; i++) {
      if (a[i] && a[i].constructor == Array) {
        pushLoop(a[i]);
      } else {
        flatArray.push(a[i]);
      }
    }
  }

  pushLoop(arr);
  return flatArray;
}

var flatten = function(a, shallow, r) {

  // if r does not exist create.
  if (!r) {
    r = []
  }

  // if shallow exists concat the arrays
  if (shallow) {

    // concat joins two or more arrays as paramenters.
    // The prototype apply allows us to define the parameters
    // as an array.  Esentially concat the arrays in array a.
    return r.concat.apply(r, a);
  }


  for (var i = 0; i < a.length; i++) {
    // Only flatten items that are arrays and push everything
    // else to the end.
    if (a[i].constructor == Array) {
      flatten(a[i], shallow, r);
    } else {
      r.push(a[i]);
    }
  }
  return r;
}

var output = document.getElementById("output");

var arr = (flatten([1, [2],
  [3, [
    [4]
  ]],
  [5, 6]
]));

output.innerHTML = JSON.stringify(arr);