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Function: unless()

Call Signature​

unless<T>(predicate, fn): (value) => T

Defined in: function/unless/index.ts:50

Creates a function that conditionally applies a transformation (inverse of when).

Returns a function that tests its input against the predicate. If the predicate fails, applies the given function; otherwise returns the input unchanged.

This is the inverse of when - it applies the transformation when the condition is NOT met.

Type Parameters​

T​

T

Parameters​

predicate​

Predicate<T>

fn​

Mapper<T, T>

Returns​

(value): T

Parameters​

value​

T

Returns​

T

Examples​

const ensureArray = unless(
  Array.isArray,
  (value) => [value]
)

ensureArray([1, 2, 3])  // => [1, 2, 3]
ensureArray(5)          // => [5]
ensureArray('hello')    // => ['hello']
// Data-first
const result = unless(
  (str: string) => str.startsWith('http'),
  (str) => `https://${str}`,
  'example.com'
)
// => 'https://example.com'
// In a pipe
pipe(
  getConfig(),
  unless(
    (config) => 'apiKey' in config,
    (config) => ({ ...config, apiKey: process.env.API_KEY })
  )
)

See​

when - for the normal condition

Call Signature​

unless<T>(predicate, fn, value): T

Defined in: function/unless/index.ts:51

Creates a function that conditionally applies a transformation (inverse of when).

Returns a function that tests its input against the predicate. If the predicate fails, applies the given function; otherwise returns the input unchanged.

This is the inverse of when - it applies the transformation when the condition is NOT met.

Type Parameters​

T​

T

Parameters​

predicate​

Predicate<T>

fn​

Mapper<T, T>

value​

T

Returns​

T

Examples​

const ensureArray = unless(
  Array.isArray,
  (value) => [value]
)

ensureArray([1, 2, 3])  // => [1, 2, 3]
ensureArray(5)          // => [5]
ensureArray('hello')    // => ['hello']
// Data-first
const result = unless(
  (str: string) => str.startsWith('http'),
  (str) => `https://${str}`,
  'example.com'
)
// => 'https://example.com'
// In a pipe
pipe(
  getConfig(),
  unless(
    (config) => 'apiKey' in config,
    (config) => ({ ...config, apiKey: process.env.API_KEY })
  )
)

See​

when - for the normal condition