string.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

referenceStr.localeCompare(compareString[, locales[, options]])

Parameters

Check the Browser compatibility section to see which browsers support the locales and options arguments, and the Checking for support for locales and options arguments for feature detection.

compareString

The string against which the referring string is compared

locales

Optional. A string with a BCP 47 language tag, or an array of such strings. For the general form and interpretation of the locales argument, see the Intl page. The following Unicode extension keys are allowed:

co

Variant collations for certain locales. Possible values include: "big5han", "dict", "direct", "ducet", "gb2312", "phonebk", "phonetic", "pinyin", "reformed", "searchjl", "stroke", "trad", "unihan". The "standard" and "search" values are ignored; they are replaced by the options property usage (see below).

kn

Whether numeric collation should be used, such that "1" < "2" < "10". Possible values are "true" and "false". This option can be set through an options property or through a Unicode extension key; if both are provided, the options property takes precedence.

kf

Whether upper case or lower case should sort first. Possible values are "upper", "lower", or "false" (use the locale's default). This option can be set through an options property or through a Unicode extension key; if both are provided, the options property takes precedence.

options

Optional. An object with some or all of the following properties:

localeMatcher

The locale matching algorithm to use. Possible values are "lookup" and "best fit"; the default is "best fit". For information about this option, see the Intl page.

usage

Whether the comparison is for sorting or for searching for matching strings. Possible values are "sort" and "search"; the default is "sort".

sensitivity

Which differences in the strings should lead to non-zero result values. Possible values are:

• "base": Only strings that differ in base letters compare as unequal. Examples: a ≠ b, a = á, a = A.
• "accent": Only strings that differ in base letters or accents and other diacritic marks compare as unequal. Examples: a ≠ b, a ≠ á, a = A.
• "case": Only strings that differ in base letters or case compare as unequal. Examples: a ≠ b, a = á, a ≠ A.
• "variant": Strings that differ in base letters, accents and other diacritic marks, or case compare as unequal. Other differences may also be taken into consideration. Examples: a ≠ b, a ≠ á, a ≠ A.

The default is "variant" for usage "sort"; it's locale dependent for usage "search".

ignorePunctuation

Whether punctuation should be ignored. Possible values are true and false; the default is false.

numeric

Whether numeric collation should be used, such that "1" < "2" < "10". Possible values are true and false; the default is false. This option can be set through an options property or through a Unicode extension key; if both are provided, the options property takes precedence. Implementations are not required to support this property.

caseFirst

Whether upper case or lower case should sort first. Possible values are "upper", "lower", or "false" (use the locale's default); the default is "false". This option can be set through an options property or through a Unicode extension key; if both are provided, the options property takes precedence. Implementations are not required to support this property.

Return value

A negative number if the reference string occurs before the compare string; positive if the reference string occurs after the compare string; 0 if they are equivalent.

Description

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

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

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.

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 a case-insensitive sort of an array.

var items = ['réservé', 'premier', 'cliché', 'communiqué', 'café', 'adieu'];
items.sort((a, b) => a.localeCompare(b)); // ['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

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.

Specifications

Browser compatibility

See also

• Intl.Collator