jQuery addClass() 源码解读

    addClass: function( value ) {
        var classes, elem, cur, clazz, j,
            i = 0,
            len = this.length,
            proceed = typeof value === "string" && value;

        if ( jQuery.isFunction( value ) ) {
            return this.each(function( j ) {
                jQuery( this ).addClass( value.call( this, j, this.className ) );
            });
        }

        if ( proceed ) {//如果传入的值不是字符串
            // The disjunction here is for better compressibility (see removeClass)
            //如果传入的是假值，"".match()
            //core_rnotwhite 用于匹配非空格字符，且有g标识,匹配成功返回数组，匹配不成功返回[]
            classes = ( value || "" ).match( core_rnotwhite ) || [];
            //rclass = /[\t\r\n\f]/g
            for ( ; i < len; i++ ) {
                elem = this[ i ];
                //如果elem是元素，cur会返回真值，" "或者 " a b "
                //替换tab符 回车 换行 换页
                cur = elem.nodeType === 1 && ( elem.className ?
                    ( " " + elem.className + " " ).replace( rclass, " " ) :
                    " "
                );
                if ( cur ) {
                    j = 0;
                    //遍历，如果原始class中不存在，字符串拼接
                    while ( (clazz = classes[j++]) ) {
                        if ( cur.indexOf( " " + clazz + " " ) < 0 ) {
                            cur += clazz + " ";
                        }
                    }
                    elem.className = jQuery.trim( cur );//最后两边去空格

                }
            }
        }

        return this;
    }

removeClass、hasClass与addClass代码类似。