Documentation / @eventkit/base / first
Function: first()
Call Signature
function first<T, D>(predicate?, defaultValue?): SingletonOperatorFunction<T, T | D>Emits the first value emitted by the source observable that satisfies a specified condition. If no such value is found when the source observable completes, the defaultValue is emitted if it's provided. If it isn't, a NoValuesError is thrown.
Type Parameters
| Type Parameter | Default type |
|---|---|
T | - |
D | T |
Parameters
| Parameter | Type | Description |
|---|---|---|
predicate? | null | A function that evaluates each value emitted by the source observable. Returns true if the value satisfies the condition, false otherwise. |
defaultValue? | D | The default value returned when no value matches the predicate. |
Returns
SingletonOperatorFunction<T, T | D>
Throws
Will throw a NoValuesError if no value is found and no default value is provided.
Call Signature
function first<T>(predicate): SingletonOperatorFunction<T, TruthyTypesOf<T>>Emits the first value emitted by the source observable that satisfies a specified condition. If no such value is found when the source observable completes, the defaultValue is emitted if it's provided. If it isn't, a NoValuesError is thrown.
Type Parameters
| Type Parameter |
|---|
T |
Parameters
| Parameter | Type | Description |
|---|---|---|
predicate | BooleanConstructor | A function that evaluates each value emitted by the source observable. Returns true if the value satisfies the condition, false otherwise. |
Returns
SingletonOperatorFunction<T, TruthyTypesOf<T>>
Throws
Will throw a NoValuesError if no value is found and no default value is provided.
Call Signature
function first<T, D>(predicate, defaultValue): SingletonOperatorFunction<T, D | TruthyTypesOf<T>>Emits the first value emitted by the source observable that satisfies a specified condition. If no such value is found when the source observable completes, the defaultValue is emitted if it's provided. If it isn't, a NoValuesError is thrown.
Type Parameters
| Type Parameter |
|---|
T |
D |
Parameters
| Parameter | Type | Description |
|---|---|---|
predicate | BooleanConstructor | A function that evaluates each value emitted by the source observable. Returns true if the value satisfies the condition, false otherwise. |
defaultValue | D | The default value returned when no value matches the predicate. |
Returns
SingletonOperatorFunction<T, D | TruthyTypesOf<T>>
Throws
Will throw a NoValuesError if no value is found and no default value is provided.
Call Signature
function first<T, S>(predicate, defaultValue?): SingletonOperatorFunction<T, S>Emits the first value emitted by the source observable that satisfies a specified condition. If no such value is found when the source observable completes, the defaultValue is emitted if it's provided. If it isn't, a NoValuesError is thrown.
Type Parameters
| Type Parameter |
|---|
T |
S |
Parameters
| Parameter | Type | Description |
|---|---|---|
predicate | (value, index) => value is S | A function that evaluates each value emitted by the source observable. Returns true if the value satisfies the condition, false otherwise. |
defaultValue? | S | The default value returned when no value matches the predicate. |
Returns
SingletonOperatorFunction<T, S>
Throws
Will throw a NoValuesError if no value is found and no default value is provided.
Call Signature
function first<T, S, D>(predicate, defaultValue): SingletonOperatorFunction<T, S | D>Emits the first value emitted by the source observable that satisfies a specified condition. If no such value is found when the source observable completes, the defaultValue is emitted if it's provided. If it isn't, a NoValuesError is thrown.
Type Parameters
| Type Parameter |
|---|
T |
S |
D |
Parameters
| Parameter | Type | Description |
|---|---|---|
predicate | (value, index) => value is S | A function that evaluates each value emitted by the source observable. Returns true if the value satisfies the condition, false otherwise. |
defaultValue | D | The default value returned when no value matches the predicate. |
Returns
SingletonOperatorFunction<T, S | D>
Throws
Will throw a NoValuesError if no value is found and no default value is provided.
Call Signature
function first<T, D>(predicate, defaultValue?): SingletonOperatorFunction<T, T | D>Emits the first value emitted by the source observable that satisfies a specified condition. If no such value is found when the source observable completes, the defaultValue is emitted if it's provided. If it isn't, a NoValuesError is thrown.
Type Parameters
| Type Parameter | Default type |
|---|---|
T | - |
D | T |
Parameters
| Parameter | Type | Description |
|---|---|---|
predicate | (value, index) => boolean | A function that evaluates each value emitted by the source observable. Returns true if the value satisfies the condition, false otherwise. |
defaultValue? | D | The default value returned when no value matches the predicate. |
Returns
SingletonOperatorFunction<T, T | D>
Throws
Will throw a NoValuesError if no value is found and no default value is provided.