String.prototype.padStart()

The padStart() method pads the current string with another string (multiple times, if needed) until the resulting string reaches the given length. The padding is applied from the start of the current string.

Syntax

str.padStart(targetLength [, padString])

Parameters

targetLength

The length of the resulting string once the current string has been padded. If the value is less than the current string's length, the current string is returned as is.

padString Optional

The string to pad the current string with. If this padding string is too long to stay within the targetLength, it will be truncated from the end. The default value is " " (U+0020 'SPACE').

Return value

A String of the specified length with the pad string applied from the start.

Example 1

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

Example 2

// Javascript version of: (unsigned)
//  printf "%0*d" width num
function leftFillNum(num, targetLength) {
    return num.toString().padStart(targetLength, 0);
}

const num = 123;
console.log(leftFillNum(num, 5));
// expected output: "00123"

Polyfill

Running the following code before any other code will create String.prototype.padStart() if it's not natively available.

// https://github.com/uxitten/polyfill/blob/master/string.polyfill.js
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart
if (!String.prototype.padStart) {
    String.prototype.padStart = function padStart(targetLength, padString) {
        targetLength = targetLength >> 0; //truncate if number, or convert non-number to 0;
        padString = String(padString !== undefined ? padString : ' ');
        if (this.length >= targetLength) {
            return String(this);
        } else {
            targetLength = targetLength - this.length;
            if (targetLength > padString.length) {
                padString += padString.repeat(targetLength / padString.length); //append to original to ensure we are longer than needed
            }
            return padString.slice(0, targetLength) + String(this);
        }
    };
}

Specifications

Browser compatibility

See also

• String.prototype.padEnd()