在JavaScript中實現數字格式化可以通過以下代碼示例：

function number_format(number, decimals, dec_point, thousands_sep) {
    number = parseFloat(number);

    if(isNaN(number) || !isFinite(number)) {
        return number;
    }

    decimals = decimals || 0;
    dec_point = dec_point || '.';
    thousands_sep = thousands_sep || ',';

    var roundedNumber = Math.round(Math.abs(number) * Math.pow(10, decimals)) / Math.pow(10, decimals);
    var roundedString = roundedNumber.toString();
    var decIndex = roundedString.indexOf('.');
    var integerPart = decIndex > -1 ? roundedString.slice(0, decIndex) : roundedString;
    var decimalPart = decIndex > -1 ? roundedString.slice(decIndex) : '';

    var formattedNumber = integerPart.replace(/(\d)(?=(\d{3})+(?!\d))/g, "$1" + thousands_sep);

    if(decimals > 0) {
        formattedNumber += dec_point + (decimalPart + '000000').slice(2, decimals + 2);
    }

    return formattedNumber;
}

// 示例
var number = 1234567.8901;
var formattedNumber = number_format(number, 2, '.', ',');
console.log(formattedNumber); // 輸出: 1,234,567.89

在上面的代碼中，number_format函數實現了數字的格式化功能，可以指定小數點位數、小數點符號和千位分隔符。您可以根據需要調整參數和格式化規則。