201506150846_ "JavaScript authoritative Guide (sixth Edition)-performance optimization 10, lowercase to uppercase, filtering, Functions" (p162-168)

I. Authoritative guidelines

1. For class array objects, we cannot use the array method, but we can use Function.call ();

For example: Array.prototype.slice.call (arr,...);

Array.prototype.silice.call (arr,...);

Array.prototype.map.call (arr,...);

Two. Performance Optimization 10

1. var Someid = document.getElementById (' #someElem '); Change to: var docu = document, Someid = Docu.getelementbyid (' #someElem ');

2. Do not use with ();

3. Use closures carefully, especially in the loop;

4. In the data, variables, object properties and array elements of 4 access methods, the speed of the former is significantly better than the latter two;

5. Do not dig too deep in the array;

6. Use for (), or do-while to replace for-in;

7. In the loop, the loop condition and the loop variable are merged together;

8. Define an array for the HTML collection object;(recommended for high-performance JavaScript authors)

For example: function array (items) {

try {

Return

Aarray.prototype.concat.call (items);

}catch (ex) {

var i = 0,

Len = Items.length,

result = Array (len);

while (I < Len) {

Result[i] = items[i];

i++;

}

return result;

}

}

var docu = document;

var divs = Array (docu.getelementbytagname (' div '));

for (var i =0; i < divs.length;i++) {

var mydiv = docu.createlement ("div");

Docu.appendchild ("mydiv");

}

9. Don't touch the DOM!  

Crateattribute (name);

Createcomment (text);

Createdocumentfragment (); (recommended usage)

CreateElement (tagname);

createTextNode (text);

10. Modify CSS classes rather than styles

Three. Lowercase to uppercase

For example:

var str = [' A ', ' P ', ' P ', ' l ', ' e ', ' s '];

Array.map = Array.map | | function (A,SEP) {return Array.prototype.map.call (A,SEP)};
var newstr = Array.map(str,function (elem) {return elem.touppercase ()});
Console.log (NEWSTR);

Four. Filtering

For example:

var s = "Fsdgnxvhdzfgtrxzcsdightmjtioyujvdgs";
var NewS = Array.prototype.join.call (S, "");

Console.log (NewS); //f s d G N x v h d z F g t R x Z c s d i g h t M J t i o y u J v d G S

var lastests = Array.prototype.filter.call (news,function (elem) {return elem.match (/[^aeiou]/);}). Join ("");

Console.log (lastests); //fsdgnxvhdzfgtrxzcsdghtmjtyjvdgs

Note: The string is immutable, non-modifiable (more special), using the push (), sort (), reverse () method to modify the array, but invalid for the string!!

Five. Functions

1. A function that initializes an object becomes a constructor (constructor).

2. There are two ways of defining a function: A. Define statement b. expression

3. Example 1: Traversing object property names and properties

var object = {' A ': ' 3 ', ' str ': ' Hello ', ' funcion ': ' fn ', ' B ': ' 0.618 '};

function Printprops (obj) {for (var p in obj) Console.log (p + ":" + obj[p] + "\ n")};

Printprops (object); //

A:3

Str:hello

Funcion:fn

b:0.618

Example 2: Calculating the distance of two coordinates

function Distance (x1,x2,y1,y2) {

var dx = x2-x1;

var dy = y2-y1;

Return math.sqrt (DX*DX + dy*dy);

};

Distance (0,0,2.8,55); //0?

Example 3: Factorial

function factorial (n) {

if (n <= 1) {

return 1;

}else{

Return

N*factorial (n-1)

}

};

Factorial (4); //24

4. The name of the function will be a local variable inside the function;

5. Functions that do not return values sometimes become "processes";

6. Note:x can access a,b!!

For example:

function Fn_a (A,b) {

function Fn_b (x) {

return x*x;

}

Return Math.sqrt (Fn_b (a) + Fn_b (b));

}

7. Function expressions can appear anywhere in the JavaScript code

Six. Function calls

1. As a function call, as a method call, as a constructor invocation, call () and apply () indirect call;

2.

201506150846_ "JavaScript authoritative Guide (sixth Edition)-performance optimization 10, lowercase to uppercase, filtering, Functions" (p162-168)