when

The when() function applies a transformation if a predicate is true (or if a type guard succeeds). Otherwise, the input value is returned as is. Exists in immediate or curried form.

Interactive example

Syntax

Classic signatures

// Type Guard predicate
function when<
	GenericInput extends AnyValue, 
	GenericPredicatedInput extends GenericInput, 
	GenericOutput extends AnyValue | EscapeVoid
>(
	input: GenericInput, 
	ifFunction: (input: GenericInput) => input is GenericPredicatedInput, 
	theFunction: (predicatedInput: GenericPredicatedInput) => GenericOutput
): GenericOutput | Exclude<GenericInput, GenericPredicatedInput>;

// Boolean predicate
function when<
	GenericInput extends AnyValue, 
	GenericOutput extends AnyValue | EscapeVoid
>(
	input: GenericInput, 
	ifFunction: (input: GenericInput) => boolean, 
	theFunction: (predicatedInput: GenericInput
) => GenericOutput): GenericOutput | GenericInput;

Curried signatures

// Type Guard predicate
function when<
	GenericInput extends AnyValue, 
	GenericPredicatedInput extends GenericInput, 
	GenericOutput extends AnyValue | EscapeVoid
>(
	ifFunction: (input: GenericInput) => input is GenericPredicatedInput, 
	theFunction: (predicatedInput: GenericPredicatedInput) => GenericOutput
): (
	input: GenericInput
) => GenericOutput | Exclude<BreakGenericLink<GenericInput>, GenericPredicatedInput>;

// Boolean predicate
function when<
	GenericInput extends AnyValue,
	GenericOutput extends AnyValue | EscapeVoid
>(
	ifFunction: (input: GenericInput) => boolean, 
	theFunction: (predicatedInput: GenericInput) => GenericOutput
): (
	input: GenericInput
) => GenericOutput | GenericInput;

Parameters

• input : The value to test (optional in curried mode).
• ifFunction : Predicate or type guard to test the input value.
• theFunction : Function executed only if the predicate is true.

Return value

If the predicate passes, returns the result of theFunction. Otherwise, returns the input value unchanged.

See also

• whenNot - Executes when the predicate fails
• whenElse - Version with two explicit branches