利用html5特有的tag判断是否支持html5

function isSupportHtml5() {
	return !!document.createElement('canvas').getContext;
}

function isSupportCss3() {
	//创建一个 element
	var css3Div = document.createElement('div');
	//判断是否支持圆角
	if ('border-radius' in css3Div.style) {
		//赋值css3属性
		css3Div.style['border-radius'] = '3px';
		//检测css3属性赋值是否成功
		return css3Div.style['border-radius'] == '3px';
	}
	else {
		return false;
	}
}

JavaScript 扩展 String 属性，实现 endsWith、startsWith、trim、trimEnd、trimStart和toBytes。

//左匹配
String.prototype.endsWith = function (suffix) {
    return (this.substr(this.length - suffix.length) === suffix);
}
//右匹配
String.prototype.startsWith = function (prefix) {
    return (this.substr(0, prefix.length) === prefix);
}
//去前后空格
String.prototype.trim = function () {
    return this.replace(/^\s+|\s+$/g, '');
}
//去结尾空格
String.prototype.trimEnd = function () {
    return this.replace(/\s+$/, '');
}
//去前面空格
String.prototype.trimStart = function () {
    return this.replace(/^\s+/, '');
}
//转byte数组
String.prototype.toBytes = function () {
    var ch, st, re = [];
    for (var i = 0; i < this.length; i++) {
        ch = this.charCodeAt(i);  // get char   
        st = [];                 // set up "stack"  
        do {
            st.push(ch & 0xFF);  // push byte to stack  
            ch = ch >> 8;          // shift value down by 1 byte  
        }
        while (ch);
        // add stack contents to result  
        // done because chars have "wrong" endianness  
        re = re.concat(st.reverse());
    }
    // return an array of bytes  
    return re;
}

js 格式化数字，输出格式化后的字符串。

Number.prototype.format = function (format) {
    if (!format) {
        return this.toString();
    }
    var comma = ',',
                dec = '.',
                i18n = false,
                neg = v < 0;

    var v = Math.abs(this);
    if (format.substr(format.length - 2) == '/i') {
        format = format.substr(0, format.length - 2);
        i18n = true;
        comma = '.';
        dec = ',';
    }

    var hasComma = format.indexOf(comma) != -1,
                psplit = (i18n ? format.replace(/[^\d\,]/g, '') : format.replace(/[^\d\.]/g, '')).split(dec);

    if (1 < psplit.length) {
        v = v.toFixed(psplit[1].length);
    } else if (2 < psplit.length) {
        throw ('NumberFormatException: invalid format, formats should have no more than 1 period: ' + format);
    } else {
        v = v.toFixed(0);
    }

    var fnum = v.toString();

    psplit = fnum.split('.');

    if (hasComma) {
        var cnum = psplit[0],
                    parr = [],
                    j = cnum.length,
                    m = Math.floor(j / 3),
                    n = cnum.length % 3 || 3,
                    i;

        for (i = 0; i < j; i += n) {
            if (i != 0) {
                n = 3;
            }

            parr[parr.length] = cnum.substr(i, n);
            m -= 1;
        }
        fnum = parr.join(comma);
        if (psplit[1]) {
            fnum += dec + psplit[1];
        }
    } else {
        if (psplit[1]) {
            fnum = psplit[0] + dec + psplit[1];
        }
    }

    return (neg ? '-' : '') + format.replace(/[\d,?\.?]+/, fnum);
}