padStart() 方法用另一个字符串填充当前字符串(重复,如果需要的话),以便产生的字符串达到给定的长度。填充从当前字符串的开始(左侧)应用的。

语法

str.padStart(targetLength [, padString])

参数

targetLength
当前字符串需要填充到的目标长度。如果这个数值小于当前字符串的长度,则返回当前字符串本身。
padString 可选
填充字符串。如果字符串太长,使填充后的字符串长度超过了目标长度,则只保留最左侧的部分,其他部分会被截断。此参数的缺省值为 " "(U+0020)。

返回值

在原字符串开头填充指定的填充字符串直到目标长度所形成的新字符串。

示例

'abc'.padStart(10);         // "       abc"
'abc'.padStart(10, "foo");  // "foofoofabc"
'abc'.padStart(6,"123465"); // "123abc"
'abc'.padStart(8, "0");     // "00000abc"
'abc'.padStart(1);          // "abc"

 

Polyfill

如果原生环境不支持该方法,在其他代码之前先运行下面的代码,将创建 String.prototype.padStart() 方法。

// https://github.com/uxitten/polyfill/blob/master/string.polyfill.js
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart
if (!String.prototype.padStart) {
    String.prototype.padStart = function padStart(targetLength,padString) {
        targetLength = targetLength>>0; //floor if number or convert non-number to 0;
        padString = String((typeof padString !== 'undefined' ? padString : ' '));
        if (this.length > targetLength) {
            return String(this);
        }
        else {
            targetLength = targetLength-this.length;
            if (targetLength > padString.length) {
                padString += padString.repeat(targetLength/padString.length); //append to original to ensure we are longer than needed
            }
            return padString.slice(0,targetLength) + String(this);
        }
    };
}

规范

规范 状态 备注
ECMAScript Latest Draft (ECMA-262)
String.prototype.padStart
Draft 在 ECMAScript 2017 中首次被定义。
ECMAScript 2017 (ECMA-262)
String.prototype.padStart
Standard  

浏览器兼容性

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support 57Edge Full support 15Firefox Full support 48IE No support NoOpera Full support 44Safari Full support 10WebView Android Full support 57Chrome Android Full support 57Edge Mobile Full support YesFirefox Android Full support 48Opera Android Full support 44Safari iOS Full support 10Samsung Internet Android Full support 7.0nodejs Full support 8.0.0
Full support 8.0.0
Full support 7.0.0
Disabled
Disabled From version 7.0.0: this feature is behind the --harmony runtime flag.

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