equals()

function equals<T>(...constants): Parser<T[number]>

Compares the input against a list of primitive values with the strict equality operator (===). The inferred type of the parser is that of a literal type; for example, equals('red') returns a Parser<'red'>. When called with multiple arguments, the parser will succeed if the input equals to any of the provided values, and thus return a union type.

Type Parameters

Type Parameter
T extends Primitive[]

Parameters

Parameter	Type	Description
...constants	T	One or more primitive values that are compared against data with the === operator.

Returns

Parser<T[number]>

A parser function that validates the input against the provided constants.

Examples

const parseInfo = equals('info')
parseInfo('info') // => ParseSuccess<'info'>
parseInfo('error') // => ParseFailure

const parseOne = equals(1)
parseOne(1) // => ParseSuccess<1>
parseOne(2) // => ParseFailure

Commonly used in discriminated unions:

const parseResult = oneOf([
 object({
   tag: equals('success')
 }),
 object({
   tag: equals('error')
  }),
])

If you pass in multiple values, the parser will validate a union type:

const parseLogLevel = equals('debug', 'info', 'warning', 'error')
parseLogLevel('info') // => ParseSuccess<'debug' | 'info' | 'warning' | 'error'>
parseLogLevel('error') // => ParseSuccess<'debug' | 'info' | 'warning' | 'error'>

When explicitly annotating unions, provide a tuple of the union members as type argument:

const parseLogLevel = equals<['debug', 'info', 'warning', 'error']>('debug', 'info', 'warning', 'error')

If you want a type alias, do it as such:

type LogLevelArray = ['debug', 'info', 'warning', 'error']
type LogLevel = LogLevelArray[number]
const parseLogLevel = equals<LogLevelArray>('debug', 'info', 'warning', 'error')