@stdlib/string-pad

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pad

Pad a string.

Installation

npm install @stdlib/string-pad

Usage

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

pad( str, len[, options] )

Pads a string such that the padded string has a length of len.

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

The function accepts the following options:

• lpad: string used to left pad. Default: ''.
• rpad: string used to right pad. Default: ' '.
• centerRight: boolean indicating whether to center right in the event of a tie. Default: false (i.e., center left).

By default, an input string is padded with spaces. To pad with a different character or sequence of characters, provide a pad string.

var str = pad( 'a', 10, {
    'lpad': 'b'
});
// returns 'bbbbbbbbba'

str = pad( 'a', 12, {
    'rpad': 'b'
});
// returns 'abbbbbbbbbbb'

To center an input string, provide both lpad and rpad options.

var opts = {
    'lpad': 'a',
    'rpad': 'c'
};

var str = pad( 'b', 11, opts );
// returns 'aaaaabccccc'

When both lpad and rpad are specified and len-str.length is odd, left and right padding cannot equally split the available padding space. By default, right padding receives the extra character (i.e., the input string is left-centered).

var opts = {
    'lpad': 'a',
    'rpad': 'c'
};

var str = pad( 'b', 10, opts );
// returns 'aaaabccccc'

To center right, set the centerRight option.

var opts = {
    'lpad': 'a',
    'rpad': 'c',
    'centerRight': true
};

var str = pad( 'b', 10, opts );
// returns 'aaaaabcccc'

Notes

• In contrast to lpad and rpad, any padding which does not evenly divide available space is trimmed such that the returned string length is always len.

  var opts = {
      'lpad': 'boop',
      'rpad': 'woot'
  };
  var str = pad( 'beep', 10, opts );
  // returns 'boobeepwoo'
  

• Similarly, if len < str.length, the input string is trimmed.

  // Pad right, trim right:
  var str = pad( 'beep', 2 );
  // returns 'be'
  
  // Pad left, trim left:
  str = pad( 'beep', 2, {
      'lpad': 'b'
  });
  // returns 'ep'
  
  // Pad both, trim both:
  str = pad( 'beep', 2, {
      'lpad': '@',
      'rpad': '!'
  });
  // returns 'ee'
  
  // Pad both, trim both starting from left:
  str = pad( 'abcdef', 3, {
      'lpad': '@',
      'rpad': '!'
  });
  // returns 'cde'
  
  // Pad both, trim both starting from right:
  str = pad( 'abcdef', 3, {
      'lpad': '@',
      'rpad': '!',
      'centerRight': true
  });
  // returns 'bcd'
  

Examples

var round = require( '@stdlib/math-base-special-round' );
var randu = require( '@stdlib/random-base-randu' );
var pad = require( '@stdlib/string-pad' );

var str = 'boop';
var out;
var len;
var i;

for ( i = 0; i < 100; i++ ) {
    len = round( randu()*10.0 ) + str.length;
    out = pad( str, len, {
        'lpad': 'beep',
        'rpad': 'p'
    });
    console.log( '%s. %d. %d.', out, len, out.length );
}

See Also

• @stdlib/string-pad-cli: CLI package for use as a command-line utility.
• @stdlib/string-left-pad: left pad a string.
• @stdlib/string-right-pad: right pad a string.

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.