JS字符串补全方法padStart()和padEnd()

背景：

var t = new Date().getMonth() + 1;// t ===> 7,没有0，怎么展示成下面的样子？// 2018-07-23

解决上述问题的一个思路：

// 补0函数function(num) { ???return +num < 10 ? '0' + num : num;}

padStart(len, str)

• 根据给定长度自动在字符串的前面补充想补充的字符串（只返回修改后的字符串，不修改原字符串）
  • len 给定的长度，转换后
  • str 想补充的字符串

栗子：

var a = '7';a.padStart(2, '0');// a ===> 07

var b = 'hi';b.padStart(10, 'hello');// b ===> hellohelhi, 重复补充

var c = 'hi';c.padStart(10, '');// c ===> hi

var d = 'a';d.padStart(5, false);// falsad.padStart(5, []);// ad.padStart(5, {});// [objad.padStart(5, null);// nullad.padStart(5, function(){});// funca

实现的思路：

var a = '22'a.padStart(len, str)

伪代码：

var res = ''if (str.toString) { ???res = str.toString();}else { ???res = Object.prototype.toString.call(str);}return res;

res.repeat(len/2+1).slice(0, len - a.length) + a

padEnd()
和padStart参数一样，只是把想加的字符串加到后头。

var a = 'aaa'.padEnd(15, {});// aaa[object Obje

兼容性：

目前来看用于前端需要兼容,请看：string.polyfill.js

if (!String.prototype.padStart) { ???String.prototype.padStart = function padStart(targetLength,padString) { ???????targetLength = targetLength>>0; //truncate 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); ???????} ???};}

node.js支持到版本8

JS字符串补全方法padStart()和padEnd()

原文地址：https://www.cnblogs.com/hongrunhui/p/9370954.html