Last Updated: 3/6/2026

P.when Patterns

P.when lets you define your own logic to check if the pattern should match or not.

If the predicate function given to when returns a truthy value, then the pattern will match for this input.

Note that you can narrow down the type of your input by providing a Type Guard function to P.when.


import { match, P } from 'ts-pattern';
 
type Input = { score: number };
 
const output = match({ score: 10 })
  .with(
    {
      score: P.when((score): score is 5 => score === 5),
    },
    (input) => '😐' // input is inferred as { score: 5 }
  )
  .with({ score: P.when((score) => score < 5) }, () => '😞')
  .otherwise(() => '🙂');
 
console.log(output);
// => '🙂'

Type Guard Functions

If you pass a type guard function to P.when, TS-Pattern will use its return type to narrow the input.


const isString = (x: unknown): x is string => typeof x === 'string';
const isNumber = (x: unknown): x is number => typeof x === 'number';
 
const fn = (input: { id: number | string }) =>
  match(input)
    .with({ id: P.when(isString) }, (narrowed /* : { id: string } */) => 'yes')
    .with({ id: P.when(isNumber) }, (narrowed /* : { id: number } */) => 'yes')
    .exhaustive();