About stdlib…

We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we’ve built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.

The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.

When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.

To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!

lpad

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url]

Left pad a string.

## Installation

bash
npm install @stdlib/string-left-pad

Alternatively,

- To load the package in a website via a script tag without installation and bundlers, use the [ES Module][es-module] available on the [esm][esm-url] branch (see [README][esm-readme]).
- If you are using Deno, visit the [deno][deno-url] branch (see [README][deno-readme] for usage intructions).
- For use in Observable, or in browser/node environments, use the [Universal Module Definition (UMD)][umd] build available on the [umd][umd-url] branch (see [README][umd-readme]).
- To use as a general utility for the command line, install the corresponding [CLI package][cli-section] globally.

The [branches.md][branches-url] file summarizes the available branches and displays a diagram illustrating their relationships.

To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.

## Usage

javascript
var lpad = require( '@stdlib/string-left-pad' );

#### lpad( str, len[, pad] )

Left pads a string such that the padded string has a length of at least len.

javascript
var str = lpad( 'a', 5 );
// returns '    a'

By default, an input string is padded with a Unicode “space” character (U+0020). To pad with a different character or sequence of characters, provide a pad string.

javascript
var str = lpad( 'beep', 10, 'b' );
// returns 'bbbbbbbeep'

str = lpad( 'boop', 12, 'beep' );
// returns 'beepbeepboop'

## Notes

- An output string is not guaranteed to have a length of exactly len, but to have a length of at least len. To generate a padded string having a length equal to len

javascript
    var str = lpad( 'boop', 10, 'beep' ); // => length 12
    // returns 'beepbeepboop'

    str = str.substring( str.length-10 ); // => length 10
    // returns 'epbeepboop'

- This function differs from [String.prototype.padStart][mdn-string-padstart] in the following ways:

- The function is not guaranteed to return a string having a length exactly equal to len (as explained above).
- The function does not truncate pad (from the end) in order to ensure the returned string has length len.

To replicate [String.prototype.padStart][mdn-string-padstart] truncation behavior

javascript
    var floorb = require( '@stdlib/math-base-special-floorb' );

    function padStart( str, len, pad ) {
        var n;
        if ( len <= str.length ) {
            return str;
        }
        n = floorb( len-str.length, 1, pad.length ) + str.length;
        return pad.substring( 0, len-n ) + lpad( str, n, pad );
    }

    var str = padStart( 'boop', 10, 'beep' );
    // returns 'bebeepboop'

## Examples

javascript
var discreteUniform = require( '@stdlib/random-base-discrete-uniform' );
var lpad = require( '@stdlib/string-left-pad' );

var str = 'beep';
var i;
for ( i = 0; i < 100; i++ ) {
    console.log( lpad( str, discreteUniform( str.length, str.length+10 ), 'b' ) );
}

## CLI

## Installation

To use as a general utility, install the CLI package globally

bash
npm install -g @stdlib/string-left-pad-cli

### Usage

text
Usage: lpad [options] --len=<length> [<string>]

Options:

  -h,    --help                Print this message.
  -V,    --version             Print the package version.
         --len length          Minimum string length.
         --pad str             String used to pad. Default: ' '.
         --split sep           Delimiter for stdin data. Default: '/\\r?\\n/'.

### Notes

- If the split separator is a [regular expression][mdn-regexp], ensure that the split option is either properly escaped or enclosed in quotes.

bash
    # Not escaped...
    $ echo -n $'beep\nboop' | lpad -len 8 --split /\r?\n/

    # Escaped...
    $ echo -n $'beep\nboop' | lpad -len 8 --split /\\r?\\n/

- The implementation ignores trailing delimiters.

### Examples

bash
$ lpad beep --len 10 --pad b
bbbbbbbeep

To use as a [standard stream][standard-streams],

bash
$ echo -n 'beep' | lpad --len 8
    beep

By default, when used as a [standard stream][standard-streams], the implementation assumes newline-delimited data. To specify an alternative delimiter, set the split option.

bash
$ echo -n 'boop\tbeep' |  lpad --len 8 --split '\t'
    boop
    beep

*

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

—-

## License

See [LICENSE][stdlib-license].

## Copyright

Copyright © 2016-2025. The Stdlib [Authors][stdlib-authors].