when

Creates a function that executes an action only if the predicate is true.

Syntax

when<T extends (...args: any) => any>(
  predicate: (...args: any) => boolean,
  action: T
): (...args: Parameters<T>) => ReturnType<T> | undefined

Parameters

Name	Type	Description
predicate	(...args: any) => boolean	Function that determines if the action will be executed.
action	T	Function to be executed if the predicate returns true.

Returns

Type	Description
(...args: Parameters<T>) => ReturnType<T> | undefined	Function that executes the action only if the predicate is true, otherwise returns undefined.

Example

const isPositive = (x: number) => x > 0;
const logIfPositive = when(isPositive, x => console.log(x));
logIfPositive(5); // prints 5
logIfPositive(-2); // does nothing

Notes

• Throws a TypeError if predicate or action are not functions.
• Useful for conditional control of function execution.

References

• MDN: Conditional (ternary) operator