Learn the iterator pattern of JavaScript design patterns

Learn the iterator pattern of JavaScript design patterns_javascript skills

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Release： 2016-05-16 15:19:16

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The iterator pattern refers to providing a method to sequentially access the individual elements in an aggregate object without exposing the internal representation of the object.

Array.prototype.forEach in JavaScript

1. Iterator in jQuery

$.each([1, 2, 3], function(i, n) {
  console.log("当前下标为："+ i + " 当前元素为："+ n );
});

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2. Implement your own iterator

var each = function(ary, callback) {
  for(var i = 0, l = ary.length; i < l; i++) {
    callback.call(ary[i], i, ary[i]);
  }  
};
each([1, 2, 3], function(i, n) {
  console.log("当前下标为："+ i + " 当前元素为："+ n );
});

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Note: parameters that are different from Array.prototype.forEach! ! !

[1, 2, 3].forEach(function(n, i, curAry){
  console.log("当前下标为："+ i + " 当前元素为："+ n + " 当前数组为：" + curAry);
})

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3. Internal iterator, external iterator

(1) Internal iterator: has defined the iteration rules, it completely takes over the entire iteration process, and only requires an initial call from the outside. The above custom each is an internal iterator!
(2) External iterator: must explicitly request iteration of the next element.
Example: Determine whether two arrays are equal

Example 1: Internal iterator

// 内部迭代器
var each = function(ary, callback) {
  for(var i = 0, l = ary.length; i < l; i++) {
    callback.call(ary[i], i, ary[i]);
  }  
};
// 比较函数
var compareAry = function(ary1, ary2) {
  if(ary1.length != ary2.length) {
    throw new Error("不相等"); // return console.log("不相等"); 
  }
  // 且住
  each(ary1, function(i, n) {
    if(n !== ary2[i]) {
      // return console.log("不相等"); 
      // return 只能返回到each方法外，后续console.log("相等")会继续执行，所以这里得使用throw
      throw new Error("不相等");
    }
  });
  console.log("相等");
}

compareAry([1, 2, 3], [1, 2, 4]);

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Example 2: External iterator

// 外部迭代器
var Iterator = function(obj) {
  var current = 0,
    next = function() {
      current++;
    },
    isDone = function() {
      return current >= obj.length;  
    },
    getCurrentItem = function() {
      return obj[current];
    };
  return {
    next: next,
    isDone: isDone,
    getCurrentItem: getCurrentItem
  };
};
// 比较函数
var compareAry = function(iterator1, iterator2) {
  while( !iterator1.isDone() && !iterator2.isDone() ){
    if(iterator1.getCurrentItem() !== iterator2.getCurrentItem()) {
      throw new Error("不相等");
    }
    iterator1.next();
    iterator2.next();
  }
  console.log("相等");
}

compareAry(new Iterator([1, 2, 3]), new Iterator([1, 2, 4]));

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4. Terminate iterator

var each = function(ary, callback) {
  for(var i = 0, l = ary.length; i < l; i++) {
    if(callback.call(ary[i], i, ary[i]) === false) {
      break;
    }
  }
}

each([1, 2, 4, 1], function(i, n) {
  if(n > 3) {
    return false;
  }
  console.log(n);
});

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5. Application (implementation)

File upload, obtain the corresponding upload component object according to different browsers.
Compare "JavaScript Design Pattern – Chain of Responsibility Pattern"

var iteratorUploadObj = function() {
  for(var i = 0, fn; fn = arguments[i]; i++) {
    var uploadObj = fn();
    if(uploadObj !== false) {
      return uploadObj;
    }
  }
};

var uploadObj = iteratorUploadObj(getActiveUploadObj, getFlashUploadObj, getFormUploadObj);

function getActiveUploadObj() {
  try{
    return new ActiveObject("TXFTNActiveX.FTNUpload"); // IE上传控件
  }catch(e) {
    return false;
  }
}

function getFlashUploadObj() {
  if(supportFlash().f === 1) {
    var str = '<object type="application/x-shockwave-flash"></object>';
    return $(str).appendTo($("body"));
  }
  return false;
}

function getFormUploadObj() {
  var str = '<input name="file" type="file" class="ui-file" />';
  return $(str).appendTo($("body"));
}

// 是否支持flash
function supportFlash() {
  var hasFlash = 0; //是否安装了flash
  var flashVersion = 0; //flash版本
  if (document.all) {
    var swf = new ActiveXObject('ShockwaveFlash.ShockwaveFlash');
    if (swf) {
      hasFlash = 1;
      VSwf = swf.GetVariable("$version");
      flashVersion = parseInt(VSwf.split(" ")[1].split(",")[0]);
    }
  } else {
    if (navigator.plugins && navigator.plugins.length > 0) {
    var swf = navigator.plugins["Shockwave Flash"];
      if (swf) {
        hasFlash = 1;
        var words = swf.description.split(" ");
        for (var i = 0; i < words.length; ++i) {
          if (isNaN(parseInt(words[i]))) continue;
          flashVersion = parseInt(words[i]);
        }
      }
    }
  }
  return { f: hasFlash, v: flashVersion };
}

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I hope this article will be helpful to everyone learning JavaScript programming.