string.padStart
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
Running the following code before any other code will create String.prototype.padStart()
if it's not natively available.
// 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(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
Specification | Status | Comment |
---|---|---|
ECMAScript Latest Draft (ECMA-262)The definition of 'String.prototype.padStart' in that specification. | Living Standard | Initial definition in ECMAScript 2017. |
ECMAScript 2017 (ECMA-262)The definition of 'String.prototype.padStart' in that specification. | Standard | |
浏览器兼容性
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic Support | 57 | 15 | 48 | 15 | 44 | 10 |
Feature | Android | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
---|---|---|---|---|---|---|---|
Basic Support | ? | 57 | (Yes) | 48 | No | ? | 10 |
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