将阿拉伯数字每三位分隔，如15000->15,000

formatNumber.js

// 直白一点的方法， 循环拼接字符串
function formatNumber(number, split) {
    var string = String(number);
    var split = split || 3;
    var newStr= new Array(string.length+ parseInt(string.length/split)); 
    newStr[newStr.length-1]=string[string.length-1]; 
    var currentIndex=string.length-1; 
    for(var i = newStr.length-1;i >= 0;i--) 
    { 
        if((newStr.length-i)%(split+1)==0) 
        { 
          newStr[i]=","; 
      }
      else
      { 
          newStr[i]=string[currentIndex--]; 
      } 
  } 
  return newStr.join("") 
}

// 直白2
function formatNumber(number, split) {
    var string = String(number);
    var split = split || 3;

    var rmb='';
    var i2=0;
    for(var i=string.length-1; i>=0; i--){

        if(i%split==0&&i!=0){
            rmb+=string[i2]+',';
        }else{
            rmb+=string[i2];
        }
        i2++;
    }
    return rmb;
}

// 首先想到的是正则表达式，用到的正则为(/(\d{3})/g 或者 /(\d{3})(?=[^$])/g
function formatNumber(number, split) {
    var string = String(number);
    var split = split || 3;
    // 注意这里，动态拼接正则的时候，注意转义字符
    var SplitReg = eval("/(\\d{"+split+"})/g");
    // var SplitReg = new RegExp("(\\d{"+split+"})", "g");

    // 正则对字符串是从左向右划分，需要注意反转
    return string.split("").reverse().join("").replace(SplitReg, "$1,").split("").reverse().join("");
}

// 上面不严谨，还有直接使用的正则
'1500000000000'.replace(/\B(?=(\d{3})+$)/g,',');
'1500000000000'.replace(/(?=(?!^)(?:\d{3})+(?:\.|$))(\d{3}(\.\d+$)?)/g,',$1');

// 利用toLocaleString()方法
15000000..toLocaleString();

// 递归
function formatNumber(number, split) {
    number = String(number);
    var split = split || 3;

    if (number.length <= split) {
        return number;
    } else {
        return formatNumber(number.substr(0, number.length - split) , split) + ',' + number.substr(number.length - split);
    }
}