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

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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():沒有傳回值，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 arrayl = [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: 1l.filter(my_func)// print: f,t// return: truel.some(my_func)// print: f// return: falsel.every(my_func)// print: f,t,f,f,f//return: undefinedl.forEach(my_func)內部實現// From：http://developer.mozilla.orgArray.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