map():返回一個新的Array，每個元素為調用func的結果

filter():返回一個符合func條件的元素數組

some():返回一個boolean，判斷是否有元素是否符合func條件

every():返回一個boolean，判斷每個元素是否符合func條件

forEach():沒有返回值，只是針對每個元素調用func

API的區別

function my_func(item) {
  if (item == 1) {
    console.log(‘t‘);
    return true;
  }
  console.log(‘f‘);
  return false;
}

// init an array
l = [0,1,2,3,4]

// print: f,t,f,f,f
// return:[false, true, false, false, false]
l.map(my_func)


// print: f,t,f,f,f
// return: 1
l.filter(my_func)

// print: f,t
// return: true
l.some(my_func)

// print: f
// return: false
l.every(my_func)

// print: f,t,f,f,f
//return: undefined
l.forEach(my_func)
內部實現

// From：http://developer.mozilla.org

Array.prototype.map = function(fun /*, thisp*/)
{
  var len = this.length;
  if (typeof fun != "function")
    throw new TypeError();

  var res = new Array(len);
  var thisp = arguments[1];
  for (var i = 0; i < len; i++)
  {
    if (i in this)
      res[i] = fun.call(thisp, this[i], i, this);
  }

  return res;
};

Array.prototype.filter = function(fun /*, thisp*/)
{
  var len = this.length;
  if (typeof fun != "function")
    throw new TypeError();

  var res = new Array();
  var thisp = arguments[1];
  for (var i = 0; i < len; i++)
  {
    if (i in this)
    {
      var val = this[i]; // in case fun mutates this
      if (fun.call(thisp, val, i, this))
        res.push(val);
    }
  }

  return res;
};

Array.prototype.some = function(fun /*, thisp*/)
{
  var len = this.length;
  if (typeof fun != "function")
    throw new TypeError();

  var thisp = arguments[1];
  for (var i = 0; i < len; i++)
  {
    if (i in this && fun.call(thisp, this[i], i, this))
      return true;
  }

  return false;
};

Array.prototype.every = function(fun /*, thisp*/)
{
  var len = this.length;
  if (typeof fun != "function")
    throw new TypeError();

  var thisp = arguments[1];
  for (var i = 0; i < len; i++)
  {
    if (i in this && !fun.call(thisp, this[i], i, this))
    return false;
  }

  return true;
};

Array.prototype.forEach = function(fun /*, thisp*/)
{
  var len = this.length;
  if (typeof fun != "function")
    throw new TypeError();

  var thisp = arguments[1];
  for (var i = 0; i < len; i++)
  {
    if (i in this)
      fun.call(thisp, this[i], i, this);
  }
};
 

JS中some(),every(),forEach(),map(),filter()區別