String.prototype.padLeft = function(pad) {
        var s = Array.apply(null, Array(pad)).map(function() { return "0"; }).join('') + this;
        return s.slice(-1 * Math.max(this.length, pad));
    };

用法:

“123”.padLeft（2） 返回：“123” “12”.padLeft（2） 返回：“12” “1”.padLeft（2） 返回：“01”

还有一种结合了几个解决方案的方法:

/**
 * pad string on left
 * @param {number} number of digits to pad, default is 2
 * @param {string} string to use for padding, default is '0' *
 * @returns {string} padded string
 */
String.prototype.paddingLeft = function (b, c) {
    if (this.length > (b||2))
        return this + '';
  return (this || c || 0) + '', b = new Array((++b || 3) - this.length).join(c || 0), b + this
};

/**
 * pad string on right
 * @param {number} number of digits to pad, default is 2
 * @param {string} string to use for padding, default is '0' *
 * @returns {string} padded string
 */
String.prototype.paddingRight = function (b, c) {
  if (this.length > (b||2))
        return this + '';
  return (this||c||0) + '', b = new Array((++b || 3) - this.length).join(c || 0), this + b
};

下面是一个JavaScript函数，它使用自定义符号添加指定数量的填充。该函数接受三个参数。

padMe --> string or number to left pad
pads  --> number of pads
padSymble --> custom symbol, default is "0"

function leftPad(padMe, pads, padSymble) {
    if(typeof padMe === "undefined") {
        padMe = "";
    }
    if (typeof pads === "undefined") {
        pads = 0;
    }
    if (typeof padSymble === "undefined") {
        padSymble = "0";
    }

    var symble = "";
    var result = [];
    for(var i=0; i < pads; i++) {
       symble += padSymble;
    }
    var length = symble.length - padMe.toString().length;
    result = symble.substring(0, length);
    return result.concat(padMe.toString());
}

以下是一些结果:

> leftPad(1)
"1"

> leftPad(1, 4)
"0001"

> leftPad(1, 4, "0")
"0001"

> leftPad(1, 4, "@")
"@@@1"

基于这个问题的最佳答案，我为String做了一个名为padLeft的原型(就像我们在c#中所做的一样):

String.prototype.padLeft = function (paddingChar, totalWidth) {
    if (this.toString().length >= totalWidth)
        return this.toString();

    var array = new Array(totalWidth); 

    for (i = 0; i < array.length; i++)
        array[i] = paddingChar;

    return (array.join("") + this.toString()).slice(-array.length);
}

用法:

var str = "12345";
console.log(str.padLeft("0", 10)); //Result is: "0000012345"

小提琴

如果你只是想要一个非常简单的单行程序来填充，只需创建一个所需填充字符的字符串，以及所需的最大填充长度，然后将其子字符串转换为你想要填充的长度。

示例:用空格填充e中的字符串存储到25个字符长。

var e = "hello"; e = e + "                         ".substring(e.length)

结果:“hello”

如果你想用一个数字作为输入做同样的事情，只需在它之前调用. tostring()。

/**************************************************************************************************
Pad a string to pad_length fillig it with pad_char.
By default the function performs a left pad, unless pad_right is set to true.

If the value of pad_length is negative, less than, or equal to the length of the input string, no padding takes place.
**************************************************************************************************/
if(!String.prototype.pad)
String.prototype.pad = function(pad_char, pad_length, pad_right) 
{
   var result = this;
   if( (typeof pad_char === 'string') && (pad_char.length === 1) && (pad_length > this.length) )
   {
      var padding = new Array(pad_length - this.length + 1).join(pad_char); //thanks to http://stackoverflow.com/questions/202605/repeat-string-javascript/2433358#2433358
      result = (pad_right ? result + padding : padding + result);
   }
   return result;
}

然后你可以这样做:

alert( "3".pad("0", 3) ); //shows "003"
alert( "hi".pad(" ", 3) ); //shows " hi"
alert( "hi".pad(" ", 3, true) ); //shows "hi "