padStart

padStart is a string method in JavaScript that pads the current string on the left with another string until the resulting length reaches the specified target length. It was introduced in ECMAScript 2017 (ES8) and is defined on String.prototype. The method returns a new string and does not modify the original.

Syntax: str.padStart(targetLength [, padString]). The parameter targetLength specifies the desired total length of the output string. If targetLength is less than or equal to the input length, the original string is returned. The optional padString defaults to a single space. If padString is an empty string, no padding is added.

Behavior: The padding string is repeated as needed and truncated to fit the remaining length. Before padding, the receiver is converted to a string via ToString. If padString is provided, it is also converted to a string. This method is purely functional: it produces a new string and does not mutate the input.

Compatibility and use: padStart is supported in modern web browsers and in Node.js environments that implement ES8. It is commonly used to align numbers, create fixed-width text, or format identifiers. See also padEnd for right padding.

Examples: '5'.padStart(3, '0') yields '005'. 'foo'.padStart(10) yields ' foo' (seven spaces). '123'.padStart(6, 'abc') yields 'abc123'.