是否有一个JavaScript函数可以填充字符串以达到确定的长度？ [英] Is there a JavaScript function that can pad a string to get to a determined length?

问题描述

我需要一个JavaScript函数，它可以获取一个值并将其填充到给定的长度（我需要空格，但任何事情都可以）。我发现了这个：

I am in need of a JavaScript function which can take a value and pad it to a given length (I need spaces, but anything would do). I found this:

代码：

String.prototype.pad = function(l, s, t){
    return s || (s = " "), (l -= this.length) > 0 ? (s = new Array(Math.ceil(l / s.length)
        + 1).join(s)).substr(0, t = !t ? l : t == 1 ? 0 : Math.ceil(l / 2))
        + this + s.substr(0, l - t) : this;
};

示例：

<script type="text/javascript">
//<![CDATA[

var s = "Jonas";
document.write(
    '<h2>S = '.bold(), s, "</h2>",
    'S.pad(20, "[]", 0) = '.bold(), s.pad(20, "[]", 0), "<br />",
    'S.pad(20, "[====]", 1) = '.bold(), s.pad(20, "[====]", 1), "<br />",
    'S.pad(20, "~", 2) = '.bold(), s.pad(20, "~", 2)
);

//]]>
</script>

但我不知道它在做什么，它似乎对我不起作用。

But I have no idea what the heck it is doing and it doesn't seem to work for me.

推荐答案

更快的方法

如果您反复这样做，例如填充数组中的值，性能是一个因素，以下方法可以为您提供近乎 100x优势的速度（ jsPerf ）目前在互联网上讨论的其他解决方案。基本思想是为pad函数提供一个完全填充的空字符串，用作缓冲区。 pad函数只是附加到要添加到此预填充字符串的字符串（一个字符串concat），然后将结果切片或修剪为所需长度。

A faster method

If you are doing this repeatedly, for example to pad values in an array, and performance is a factor, the following approach can give you nearly a 100x advantage in speed (jsPerf) over other solution that are currently discussed on the inter webs. The basic idea is that you are providing the pad function with a fully padded empty string to use as a buffer. The pad function just appends to string to be added to this pre-padded string (one string concat) and then slices or trims the result to the desired length.

function pad(pad, str, padLeft) {
  if (typeof str === 'undefined') 
    return pad;
  if (padLeft) {
    return (pad + str).slice(-pad.length);
  } else {
    return (str + pad).substring(0, pad.length);
  }
}

例如，将数字填零为一个长度10位数，

For example, to zero pad a number to a length of 10 digits,

pad('0000000000',123,true);

用空格填充字符串，所以整个字符串为255个字符，

To pad a string with whitespace, so the entire string is 255 characters,

var padding = Array(256).join(' '), // make a string of 255 spaces
pad(padding,123,true);

性能测试

参见 jsPerf 测试此处。

这是比ES6 string.repeat 快2倍，如修订后的JsPerf 这里

And this is faster than ES6 string.repeat by 2x as well, as shown by the revised JsPerf here

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