filter
The filter() method selects the elements of an array that satisfy a predicate and returns a new array. It can also serve as a type guard to refine the typing of the kept elements.
Interactive example
Syntax
Classic signature
typescript
// Type Guard
function filter<
GenericInput extends readonly unknown[],
GenericOutput extends GenericInput[number]
>(
input: GenericInput,
predicate: (
element: GenericInput[number],
params: ArrayFilterParams<GenericInput>
) => element is GenericOutput
): GenericOutput[]
// Boolean
function filter<
GenericInput extends readonly unknown[]
>(
input: GenericInput,
predicate: (
element: GenericInput[number],
params: ArrayFilterParams<GenericInput>
) => boolean
): GenericInput[number][]Curried signature
typescript
// Type Guard
function filter<
GenericInput extends readonly unknown[],
GenericOutput extends GenericInput[number]
>(
predicate: (
element: GenericInput[number],
params: ArrayFilterParams<GenericInput>
) => element is GenericOutput
): (input: GenericInput) => GenericOutput[]
// Boolean
function filter<
GenericInput extends readonly unknown[]
>(
predicate: (
element: GenericInput[number],
params: ArrayFilterParams<GenericInput>
) => boolean
): (array: GenericInput) => GenericInput[number][]Helper parameters
typescript
interface ArrayFilterParams<
GenericInputArray extends readonly unknown[]
> {
index: number;
self: GenericInputArray;
}Parameters
input: The array to filter.predicate: Function applied to each element. It decides whether the element is kept and can act as a type guard.params.index: Position of the current element in the original array.params.self: The whole array (useful for comparing or inspecting a neighbor during filtering).
Return value
A new array containing only the elements for which the predicate returns true. The order is preserved and the initial array remains unchanged.