Interface WritableSignalOptions<T>

An object that stores the options of the writable signal to be created.

interface WritableSignalOptions<T> {
    equal?: false | ((value: T, prevValue: T) => unknown);
    getInitialValue?: (() => T);
    name?: string;
    onActivate?: ((value: T) => void) | ((value: T) => ((value: T) => void));
    onCleanup?: ((value: T) => void);
    onCreate?: ((value?: T) => void);
    onDeactivate?: ((value: T) => void);
    onException?: ((e: unknown, prevValue: T) => void);
    onUpdate?: ((value: T, prevValue: T) => void);
}

Type Parameters

  • T

Hierarchy (view full)

Properties

equal? getInitialValue? name? onActivate? onCleanup? onCreate? onDeactivate? onException? onUpdate?

Properties

Optionalequal

equal?: false | ((value: T, prevValue: T) => unknown)

An equality function used to check whether the value of the signal has been changed. Default is Object.is.

Param: value

A new value of the signal.

Param: prevValue

A previous value of the signal.

Returns

Truthy if the values are equal, falsy otherwise.

OptionalgetInitialValue

getInitialValue?: (() => T)

Lazily provides a value for an inactive writable signal when it is read.

Semantics:

  • Called on every read while the signal has no subscribers or active dependents.
  • Called again after the signal becomes inactive (e.g., last subscriber unsubscribed).
  • Ignored while the signal is active.
  • If set/emit is called while the signal is inactive, that update is applied immediately (including onUpdate), but the next cold read will override the stored value with getInitialValue() until activation.

Optionalname

name?: string

A name of the signal. Can be accessed inside a lifecycle function via this.

OptionalonActivate

onActivate?: ((value: T) => void) | ((value: T) => ((value: T) => void))

A function called when the first subscriber or the first active dependent signal appears.

Param: value

The current value of the signal.

Returns

A function to override onDeactivate option.

OptionalonCleanup

onCleanup?: ((value: T) => void)

A function called each time before the signal value is calculated and when the signal is going to be deactivated. Useful to cleanup subscriptions and timers created during computation.

Type declaration

    • (value): void

    • Parameters

      • value: T

        The current value of the signal.

      Returns void

OptionalonCreate

onCreate?: ((value?: T) => void)

A function called at the moment the signal is created.

Type declaration

    • (value?): void

    • Parameters

      • Optionalvalue: T

        An initial value of the signal.

      Returns void

OptionalonDeactivate

onDeactivate?: ((value: T) => void)

A function called when the last subscriber or the last active dependent signal disappears.

Type declaration

    • (value): void

    • Parameters

      • value: T

        The current value of the signal.

      Returns void

OptionalonException

onException?: ((e: unknown, prevValue: T) => void)

A function called whenever an unhandled exception occurs during the calculation of the signal value.

Type declaration

    • (e, prevValue): void

    • Parameters

      • e: unknown

        An exception.

      • prevValue: T

        The previous value of the signal.

      Returns void

OptionalonUpdate

onUpdate?: ((value: T, prevValue: T) => void)

A function called each time the signal value is updated.

Type declaration

    • (value, prevValue): void

    • Parameters

      • value: T

        The new value of the signal.

      • prevValue: T

        The previous value of the signal.

      Returns void

Settings

Member Visibility

On This Page

Properties