Javascript：在 for 循环中隐藏原型方法？

Question

So lets say I've added some prototype methods to the Array class:

Array.prototype.containsKey = function(obj) {
    for(var key in this)
        if (key == obj) return true;
    return false;
}

Array.prototype.containsValue = function(obj) {
    for(var key in this)
        if (this[key] == obj) return true;
    return false;
}

then I create an associative array and attempt to loop through it's keys:

var arr = new Array();
arr['One'] = 1;
arr['Two'] = 2;
arr['Three'] = 3;

for(var key in arr)
   alert(key);

this returns five items:

  -One
  -Two
  -Three
  -containsKey
  -containsValue

but I want (expect?) only three. Am I approaching this wrong? is there a way to "hide" the prototype methods? or should I be doing something differently?

Answer 1

You can use JavaScript's hasOwnProperty method to achieve this in the loop, like this:

for(var key in arr) {
    if (arr.hasOwnProperty(key)) {
        ...
    }
}

Reference: This YUI blog article .

Answer 2

You can achieve desired outcome from the other end by making the prototype methods not enumerable:

Object.defineProperty(Array.prototype, "containsKey", {
  enumerable: false,
  value: function(obj) {
      for(var key in this)
        if (key == obj) return true;
      return false;
    }
});

This usually works better if you have control over method definitions, and in particular if you have no control over how your code will be called by other people, which is a common assumption in library code development.

Answer 3

Javascript doesn't support associative arrays the way you think they do. http://ajaxian.com/archives/javascript-associative-arrays-considered-harmful

for (var i in .. gets all of the properties of an object (an array is just another object) which is why you're seeing the other objects you've prototyped to it.

As the article suggests you should use an object:

var assoc = {'One' : 1, 'Two' : 2};
assoc['Three'] = 3;

for(var key in assoc)
   alert(key+' => '+assoc[key]);

Answer 4

you could do this:

for(var key in arr)
{
   if (typeof(arr[key]) == "function")
      continue;
   alert(key);
}

But that's a shoddy workaround

Answer 5

Method 1 : use Object.keys (which doesn't return prototype properties) & loop

Object.keys(arr); // ['One', 'Two', 'Three']
Object.keys(arr).forEach(key => console.log(key))

Method 2 : hasOwnProperty inside a for-loop.

 for(var key in arr) {
   if (arr.hasOwnProperty(key)) {
     ...
   }
 }

Answer 6

For high-performance iteration over JavaScript arrays, use either a for or while loop. Nicholas Zakas discusses the most-performant options for iterating over arrays in his Tech Talk Speed Up Your JavaScript .

Your best bet is probably something like this:

for (var i = collection.length - 1; i >= 0; i--) {
  if (obj == collection[i]) return true;
}

This approach will be peform best for a few reasons:

• Only a single local variable is allocated
• The collection's length property is only accessed once, at the initialization of the loop
• Each iteration, a local is compared to a constant ( i >= 0 ) instead of to another variable

Answer 7

You can hide methods that added to prototype from for-in loops like this:

Object.defineProperty(Array.prototype, "containsKey", { enumerable: false });

Just after you add method. where the "containsKey" is your added method.