jQuery源码 — addClass方法
本方法定义在jQuery.prototype

addClass: function( value ) {var classNames, i, l, elem,setClass, c, cl;if ( jQuery.isFunction( value ) ) {return this.each(function( j ) {jQuery( this ).addClass( value.call(this, j, this.className) );});}if ( value && typeof value === "string" ) {classNames = value.split( rspace );for ( i = 0, l = this.length; i < l; i++ ) {elem = this[ i ];if ( elem.nodeType === 1 ) {if ( !elem.className && classNames.length === 1 ) {elem.className = value;} else {setClass = " " + elem.className + " ";for ( c = 0, cl = classNames.length; c < cl; c++ ) {if ( !~setClass.indexOf( " " + classNames[ c ] + " " ) ) {setClass += classNames[ c ] + " ";}}elem.className = jQuery.trim( setClass );}}}}return this;}

发现一个转整型的好方法： ~~x
不论x是什么，基本都不会报错，实在不行就返回0，比如~~1.5，返回1

jQuery这里也用了~，这是一种取巧，即indexOf如果返回-1，~运算完正好为0，可以进行if判断，关于单个~运算，我试了几个数，感觉公式就是 -1 * ( 原值 + 1 )

if ( !~setClass.indexOf( " " + classNames[ c ] + " " ) ) {setClass += classNames[ c ] + " ";}

我写一个通用的：

function addClass( el, className ) {var classNames = className.split(/\s/),curClass = trim(el.className),itemClass;if( !curClass && classNames.length === 1 ) {//如果el的className是空的，并且只传了一个classNamethis.className = className;} else {for( var i = 0, len = classNames.length; i < len; i++ ){curClass = ' ' + curClass + ' ';//这是为了进行下面的匹配itemClass = classNames[i];if( !~curClass.indexOf( ' ' + itemClass + ' ' )){curClass += classNames[i] + ' ';}}el.className = curClass;}}