This article answers the last question about pseudo arrays mentioned in javascript universal loop traversal method forEach.
What is a pseudo array?
An object with a length attribute that can be converted into a real array through Array.prototype.slice.
There are many such objects, the more special ones are arguments objects, and calls like getElementsByTagName, document.childNodes, etc. They all return NodeList objects, which are pseudo arrays.
We can convert the pseudo array into a real Array object through Array.prototype.slice.call(fakeArray).
Let’s look at an example:
var fakeArray01 = { 0:'a',1:'b',length:2};//This is a standard pseudo-array object
var arr01 = Array.prototype.slice.call(fakeArray01);
alert( arr01[0]);//a
var arr02 = [].slice.call(fakeArray01);
alert(arr02[0]);//a
slice OK Used to obtain array fragments, it returns a new array and does not modify the original array.
In the example, you can see that fakeArray is successfully converted into an Array object. Maybe everyone is unfamiliar with the writing method of Array.prototype.slice.call. In fact, we can also achieve the same effect through [].slice.call. So why should we implement it through prototype? The answer is prototype The form executes the program more efficiently, and the code is also more beautiful.
Implementation of pseudo array
Let us take a deeper look at the implementation of pseudo array. Let’s look at some special use cases:
var fakeArray01 = {a:'a',b:'b',length:2};//There is no value corresponding to the length subscript
var arr01 = Array.prototype.slice.call(fakeArray01);
alert(arr01[0]);//undefined
var fakeArray02 = {0:'a',1:'b',length:'num'};//length is not a numerical value
var arr02 = Array.prototype.slice.call(fakeArray02);
alert(arr02[1]);//undefined
Similarly fakeArray01 and fakeArray02 are converted into real arrays, but the The values are all undefined
Looking at the source code of V8 engine
array.js , you can simplify the internal implementation of slice to:
function slice(start, end) {
var len = ToUint32(this.length ), result = [];
for(var i = start; i < end; i ) {
result.push(this[i]);
}
return result;
}
It can be seen that slice does not need this to be an array type, it only needs to have a length attribute. And the length attribute does not need to be of type number. When it cannot be converted to a numerical value, ToUnit32(this.length) returns 0.
Based on the above conclusion, it can be concluded that fakeArray01 is converted into an array with lenth of 2, and its values are all Initialized to undefined, fakeArray02 is converted into an array with length 0. Natural access to the element with subscript 1 returns undefined
IE problem
The standard browser slice implementation can already explain all the problems, but IE A problem occurred while handling NodeList. NodeList cannot be converted to a real array in IE and an error will occur. Why is this? Strictly speaking, an abstract class Arraioid is defined inside IE. Array and Arguments both inherit from this, so slice can be used. However, the DOM object is connected to JScript through COM, and slice detection fails.
Jquery and pseudo-arrays
Jquery uses pseudo-arrays extensively internally. It can be said that the entire Jquery object is built on the basis of pseudo-array, so let us look at some practical applications of Jquery:
fakeArray < ;script src="jquery-1.4.2.js" type="text/javascript">
<script> <br>$(document).ready(function(){ <br>var body = $("body"); <br>alert(body.get(0).tagName); <br>}); <br></script>
It couldn’t be a simpler program, okay, let’s take a look at its internal implementation principle:
jQuery.fn = jQuery.prototype = {
init: function( selector, context ) {
var match, elem, ret, doc;
// Handle $(""), $(null), or $(undefined)
if ( !selector ) {
return this;
}
// Handle $(DOMElement)
if ( selector.nodeType ) {
this.context = this[0] = selector;
this.length = 1;
return this;
}
// The body element only exists once, optimize finding it
if ( selector === "body" && !context ) {
this.context = document;
this[0] = document.body;
this.selector = "body ";
this.length = 1;
return this;
}
//... ...
},
get: function( num ) {
return num == null ?
// Return a 'clean' array
this.toArray() :
// Return just the object
( num < 0 ? this.slice(num) [ 0 ] : this[ num ] );
}
}
Finally, let’s explain the execution details of the program. But before that, we have to talk about Jquery Something inside.
Users who have used Jquery should all know the $() function, which is the selector representative of Jquery. We may use the $() function to select elements on the page (the specific syntax can be parameterized in the Jquery help document). In fact, when we execute the $() function, the program executes the init method listed above. Let's take a look at the events that occur when $(document) is called:
//$(document)
init: function( selector, context) {
var match, elem, ret, doc;
// Handle $(DOMElement): Handle DOM elements,
if ( selector.nodeType ) {
this.context = this[0] = selector; //Assign selector to attribute 0 Value, this time is the document object
this.length = 1; //Create a pseudo array and update the subscript
return this; //Return the Jquery object
}
//... .. .
}
$("body") is the same, so I won’t say more.
We know that all operations in Jquery return Jquery objects, so how do we get the corresponding DOM object? Jquery provides us with a get method, which is specially used to obtain DOM objects from jquery objects. Used, from this, there is body.get(0), so why is it get(0) instead of get(), because all operations of Jquery are performed on arrays. Therefore, in the get method, we need to pass a subscript value to get the specific element. Now it’s time to look at the specific implementation of the get method:
get: function( num ) {
return num == null ?
//If there is no num, return the DOM array directly
this.toArray():
//If the specified num is specified, return the specified The subscripted element
//this.slice is another method of jquery. It actually calls Array.prototype.slice internally to convert the pseudo array into a real array
( num < 0 ? this.slice (num)[ 0 ] : this[ num ] );
}
That’s it for pseudo-arrays, I think it’s almost done.
Note: If there is a chance, there may be a "Beyond Jquery" series in the future, specifically analyzing the internal implementation details of Jquery. However, since there are various evil ways inside Jquery that are not well understood, this is a problem for the future.
Reference:
http://lifesinger.org/blog/2010/05/array-prototype-slice/
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