这两种解决方案的关键技巧是创建具有给定大小(比所需长度大一个)的数组实例，然后立即调用join()方法来生成字符串。join()方法被传递填充字符串(可能是空格)。由于数组是空的，在将数组连接到一个结果字符串的过程中，空单元格将被呈现为空字符串，只有填充将保留。这是一个很好的技巧。

@Daniel LaFavers回答的一个变体。

var mask = function (background, foreground) {
  bg = (new String(background));
  fg = (new String(foreground));
  bgl = bg.length;
  fgl = fg.length;
  bgs = bg.substring(0, Math.max(0, bgl - fgl));
  fgs = fg.substring(Math.max(0, fgl - bgl));
  return bgs + fgs;
};

例如:

mask('00000', 11  );   // '00011'
mask('00011','00' );   // '00000'
mask( 2     , 3   );   // '3'
mask('0'    ,'111');   // '1'
mask('fork' ,'***');   // 'f***'
mask('_____','dog');   // '__dog'

现在是2014年，我建议使用JavaScript字符串填充函数。哈!

基本的:带空格的右垫

function pad (str, length) {
    var padding = (new Array(Math.max(length - str.length + 1, 0))).join(" ");
    return str + padding;
}

花式:选项垫

/**
 * @param {*}       str                         Input string, or any other type (will be converted to string)
 * @param {number}  length                      Desired length to pad the string to
 * @param {Object}  [opts]
 * @param {string}  [opts.padWith=" "]          Character to use for padding
 * @param {boolean} [opts.padLeft=false]        Whether to pad on the left
 * @param {boolean} [opts.collapseEmpty=false]  Whether to return an empty string if the input was empty
 * @returns {string}
 */
function pad(str, length, opts) {
    var padding = (new Array(Math.max(length - (str + "").length + 1, 0))).join(opts && opts.padWith || " "),
        collapse = opts && opts.collapseEmpty && !(str + "").length;
    return collapse ? "" : opts && opts.padLeft ? padding + str : str + padding;
}

使用(的):

pad("123", 5);
// Returns "123  "

pad(123, 5);
// Returns "123  " - non-string input

pad("123", 5, { padWith: "0", padLeft: true });
// Returns "00123"

pad("", 5);
// Returns "     "

pad("", 5, { collapseEmpty: true });
// Returns ""

pad("1234567", 5);
// Returns "1234567"

I think its better to avoid recursion because its costly. function padLeft(str,size,padwith) { if(size <= str.length) { // not padding is required. return str; } else { // 1- take array of size equal to number of padding char + 1. suppose if string is 55 and we want 00055 it means we have 3 padding char so array size should be 3 + 1 (+1 will explain below) // 2- now join this array with provided padding char (padwith) or default one ('0'). so it will produce '000' // 3- now append '000' with orginal string (str = 55), will produce 00055 // why +1 in size of array? // it is a trick, that we are joining an array of empty element with '0' (in our case) // if we want to join items with '0' then we should have at least 2 items in the array to get joined (array with single item doesn't need to get joined). // <item>0<item>0<item>0<item> to get 3 zero we need 4 (3+1) items in array return Array(size-str.length+1).join(padwith||'0')+str } } alert(padLeft("59",5) + "\n" + padLeft("659",5) + "\n" + padLeft("5919",5) + "\n" + padLeft("59879",5) + "\n" + padLeft("5437899",5));

ECMAScript 2017在String原型中增加了一个padStart方法。这个方法将用空格填充字符串到给定的长度。此方法还接受一个可选字符串，该字符串将用于代替空格作为填充。

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

还添加了以相同方式工作的padEnd方法。

关于浏览器兼容性(和一个有用的填充)，请参见此链接。

包括所有选项

function padding(stringToBePadded, paddingCharacter, totalLength, padLeftElseRight){
    //will pad any string provided in first argument, with padding character provide in 2nd argument and truncate to lenght provided in third argument, padding left if 4th argument true or undefined, right if false. 
    // i.e. padding("lode","x","10")  -->  "xxxxxxlode"
    // i.e. padding("lode","x","10",true)  -->  "xxxxxxlode"
    // i.e. padding("lode","x","10",false)  -->  "lodexxxxxx"
    // i.e. padding("12","0","5")  -->  "00012"
    {
        padLeftElseRight = typeof padLeftElseRight !== 'undefined' ? padLeftElseRight : true;
    }
    if (stringToBePadded.length > totalLength){
        // console.log("string too long to be padded");
        return stringToBePadded;
    }
    var paddingString = paddingCharacter.repeat(totalLength);//make long string of padding characters
    if ( padLeftElseRight){
        return String(paddingString+stringToBePadded).slice(-totalLength);
    }else{ 
        return String(stringToBePadded+paddingString).slice(0,totalLength); 
    }
}