Array.prototype.slice using extension_javascript tips-JS Tutorial-php.cn

Array.prototype.slice using extension_javascript tips

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Release： 2016-05-16 18:25:28

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In addition to normal usage, slice is often used to convert array-like objects into true arrays.

Noun explanation: array-like object – an object with a length attribute, such as { 0: 'foo', length: 1 } , or even { length: 'bar' }. The most common array-like objects are arguments and NodeList.

Looking at the source code of V8 engine array.js, the internal implementation of slice can be simplified to:

Copy code The code is as follows:

 
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 numeric value, ToUnit32(this.length) returns 0.

For standard browsers, the principle of slice has been clearly explained above. But the annoying ie always causes us trouble:

Copy code The code is as follows:

 
 var slice = Array.prototype.slice; 
slice.call(); // => IE: Object expected. 
slice.call(document.childNodes); // => IE: JScript object expected . 

The above code reports an error in IE. It’s a shame that IE’s Trident engine is not open source, so we can only guess:

Copy the code The code is as follows:

 
function ie_slice(start, end) { 
var len = ToUint32(this.length), result = []; 

if(__typeof__ this !== 'JScript Object') throw 'JScript object expected'; 
if(this === null) throw 'Oject expected'; 

for(var i = start; i < end; i ) { 
result.push(this[ i]); 
} 
return result; 
} 

At this point, the wretched ie is justified.

Regarding slices, there is another topic: Should I use Array.prototype.slice or [].slice? Theoretically, [] needs to create an array, and the performance will be slightly worse than Array.prototype. But in fact, the two are almost the same, just like whether to use i or i in a loop, it is purely a matter of personal habit.

The last topic is about performance. For array filtering, there is a way to write at the expense of hue:

Copy the code The code is as follows:

 
var ret = []; 
for(var i = start, j = 0; i < end; i ) { 
ret[j ] = arr[i]; 
} 
 

Exchange space for time. Remove push, and the performance improvement is quite obvious for large arrays.

Writing a blog early in the morning, I am not in a good mood, so I have to leave a topic for everyone:

Copy code The code is as follows:

 
var slice = Array.prototype.slice; 
alert(slice.call({0: 'foo', length: 'bar'})[0]); / / ? 
alert(slice.call(NaN).length); // ? 
alert(slice.call({0: 'foo', length: '100'})[0]); // ? 