How can I format an integer to a specific length in javascript?

Question

I have a number in Javascript, that I know is less than 10000 and also non-negative. I want to display it as a four-digit number, with leading zeroes. Is there anything more e

Answer 1

I know this question is kind of old, but for anyone looking for something similar to String formatting on Java or Python, I have these helper methods:

String.format = (...args) => {
    if( args.length == 0 ){
        throw new Error("String format error: You must provide at least one argument");
    }
    const delimiter = "@LIMIT*";
    const format = String(args.shift(1,0)).replace(/(%[0-9]{0,}[sd])/g, delimiter+"$1"+delimiter).split(delimiter); // First element is the format
    if( [...format].filter(el=>el.indexOf("%")>-1).length != args.length ){
        throw new Error("String format error: Arguments must match pattern");
    }
    if( format.length == 1 && args.length == 0 ){
        return String(format);
    }
    let formattedString = "";
    // patterns
    const decimalPattern = /%[0-9]{0,}d/;
    const stringPattern  = /%s/;
    if( format.length == 0 ){
        throw new Error("String format error: Invalid format");
    }
    let value        = null;
    let indexOfParam = 0;
    let currPattern  = null;
    while( args.length > 0 ) {
        currPattern = format[indexOfParam];
        indexOfParam++;
        if( currPattern.indexOf("%")<0 ){
            formattedString+=currPattern;
            continue;
        }
        value = args.shift(0,1);
        if( decimalPattern.test(currPattern) ){
            let numberLength = parseInt(currPattern.replace(/[^0-9]/g,''));
            if( isNaN(numberLength) ){
                numberLength = 0;
            }
            formattedString+=numberToLength(value, numberLength);
        } else if( stringPattern.test(currPattern) ) {
            if( typeof value === 'object' && value.toSource ){
                formattedString+=String(value.toSource());
            } else {
                formattedString+=String(value);
            }
        } else {
            throw new Error("String format error: Unrecognized pattern:"+currPattern);
        }
    }
    return formattedString;
}

const numberToLength = (number, length) => {
    length = parseInt(length);
    number = String(number);
    if( isNaN(length) || isNaN(parseInt(number)) ){
        throw new Error("Invalid number passed");
    }
    while( number.length < length ) {
        number = "0" + number;
    }
    return number;
}