String.prototype.localeCompare()

The localeCompare() method returns a number indicating whether a reference string comes before, or after, or is the same as the given string in sort order.

The new locales and options arguments let applications specify the language whose sort order should be used and customize the behavior of the function. In older implementations, which ignore the locales and options arguments, the locale and sort order used are entirely implementation-dependent.

Syntax

localeCompare(compareString)
localeCompare(compareString, locales)
localeCompare(compareString, locales, options)

Parameters

compareString

The string against which the referenceStr is compared.

locales and options

These arguments customize the behavior of the function and let applications specify the language whose formatting conventions should be used. In implementations which ignore the locales and options arguments, the locale used and the form of the string returned are entirely implementation-dependent.

See the Intl.Collator() constructor for details on these parameters and how to use them.

Return value

A negative number if referenceStr occurs before compareString; positive if the referenceStr occurs after compareString; 0 if they are equivalent.

Description

Returns an integer indicating whether the referenceStr comes before, after or is equivalent to the compareString.

  • Negative when the referenceStr occurs before compareString
  • Positive when the referenceStr occurs after compareString
  • Returns 0 if they are equivalent

Warning: Do not rely on exact return values of -1 or 1!

Negative and positive integer results vary between browsers (as well as between browser versions) because the W3C specification only mandates negative and positive values. Some browsers may return -2 or 2, or even some other negative or positive value.

Performance

When comparing large numbers of strings, such as in sorting large arrays, it is better to create an Intl.Collator object and use the function provided by its compare property.

Examples

Using localeCompare()

// The letter "a" is before "c" yielding a negative value
'a'.localeCompare('c'); // -2 or -1 (or some other negative value)

// Alphabetically the word "check" comes after "against" yielding a positive value
'check'.localeCompare('against'); // 2 or 1 (or some other positive value)

// "a" and "a" are equivalent yielding a neutral value of zero
'a'.localeCompare('a'); // 0

Sort an array

localeCompare() enables case-insensitive sorting for an array.

let items = ['réservé', 'Premier', 'Cliché', 'communiqué', 'café', 'Adieu'];
items.sort( (a, b) => a.localeCompare(b, 'fr', { ignorePunctuation: true }));
// ['Adieu', 'café', 'Cliché', 'communiqué', 'Premier', 'réservé']

Check browser support for extended arguments

The locales and options arguments are not supported in all browsers yet.

To check whether an implementation supports them, use the "i" argument (a requirement that illegal language tags are rejected) and look for a RangeError exception: 

function localeCompareSupportsLocales() {
  try {
    'foo'.localeCompare('bar', 'i');
  } catch (e) {
    return e.name === 'RangeError';
  }
  return false;
}

Using locales

The results provided by localeCompare() vary between languages. In order to get the sort order of the language used in the user interface of your application, make sure to specify that language (and possibly some fallback languages) using the locales argument: 

console.log('ä'.localeCompare('z', 'de')); // a negative value: in German, ä sorts before z
console.log('ä'.localeCompare('z', 'sv')); // a positive value: in Swedish, ä sorts after z

Using options

The results provided by localeCompare() can be customized using the options argument: 

// in German, ä has a as the base letter
console.log('ä'.localeCompare('a', 'de', { sensitivity: 'base' })); // 0

// in Swedish, ä and a are separate base letters
console.log('ä'.localeCompare('a', 'sv', { sensitivity: 'base' })); // a positive value

Numeric sorting

// by default, "2" > "10"
console.log("2".localeCompare("10")); // 1

// numeric using options:
console.log("2".localeCompare("10", undefined, { numeric: true })); // -1

// numeric using locales tag:
console.log("2".localeCompare("10", "en-u-kn-true")); // -1

Specifications

Specification
ECMAScript Language Specification
# sec-string.prototype.localecompare
ECMAScript Internationalization API Specification
# sup-String.prototype.localeCompare

Browser compatibility

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See also