javascript - Why I can call Array.prototype.filter() on a object which is not an array? - Stack Overflow

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filter    Creates a new array with all elements that pass
          the test implemented by the provided function.

---

>>> 'abc'.filter(function(e){return e!='b'})
TypeError: "abc".filter is not a function

>>> Array.prototype.filter.call('abc', function(e){return e!='b'})
["a", "c"]

>>> jQuery.isArray('abc')
false

filter    Creates a new array with all elements that pass
          the test implemented by the provided function.

---

>>> 'abc'.filter(function(e){return e!='b'})
TypeError: "abc".filter is not a function

>>> Array.prototype.filter.call('abc', function(e){return e!='b'})
["a", "c"]

>>> jQuery.isArray('abc')
false

• javascript

Share Improve this question Follow edited Apr 15, 2012 at 3:28 Pointy 414k6262 gold badges594594 silver badges628628 bronze badges asked Apr 15, 2012 at 3:22 kevkev 162k4949 gold badges283283 silver badges281281 bronze badges 2

• 1 No clue. Array.filter doesn't exist. Did you mean Array.prototype.filter.call? – Ry- ♦ Commented Apr 15, 2012 at 3:25
• @minitech Array.filter exists in Firefox. – Pointy Commented Apr 15, 2012 at 3:27

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2 Answers 2

Sorted by: Reset to default 5

All array methods are generic; i.e., they work on any object with numeric properties plus a length property. See the specification for Array.prototype.filter, for example: note that the algorithm is phrased entirely in terms of things like

Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".

or

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.

---

Array.filter is a Firefox-specific extension, BTW. The cross-browser version is Array.prototype.filter.call.

You can do this because Array.filter¹ is defined to be generic; you can call it on NodeLists, for example. This is just a convenience to the developer. It also works on strings, because in modern browsers (everything except IE 8 and earlier), you can access characters in a string like you would indices of an array, and that's what filter is based on, along with length:

var str = "Hello, world!";

str[2] // 'l'

However, I wouldn't remend making use of this feature if you plan on supporting Internet Explorer 8 or earlier, because the ES5 shivs likely won't detect the fact that the context is a string, which will break things.

_{¹ And all the other Array methods. Except, once again, on Internet Explorer 8 and earlier.}

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