仅当映射到它的类型未定义时,如何才能使arg为可选? [英] How can we make an arg be optional only if the type mapped to it is undefined?
问题描述
例如,如果我们有以下代码,
For example, if we have the following code,
type Events = {
SOME_EVENT: number
OTHER_EVENT: string
ANOTHER_EVENT: undefined
}
interface EventEmitter<EventTypes> {
on<K extends keyof EventTypes>(s: K, listener: (v: EventTypes[K]) => void);
}
declare const emitter: EventEmitter<Events>;
emitter.on('SOME_EVENT', (payload) => testNumber(payload));
emitter.on('OTHER_EVENT', (payload) => testString(payload));
function testNumber( value: number ) {}
function testString( value: string ) {}
which works (playground link), how can we make it so that calls to emit do not require the second arg for the ANOTHER_EVENT
event?
例如,我可以添加以下行,它可以正常工作:
For example, I can add the following line and it works:
emitter.emit('OTHER_EVENT', 'foo')
(但是,如果我想用'ANOTHER_EVENT'
调用emit
,我想在没有第二个arg的情况下执行此操作:
But if I want to call emit
with 'ANOTHER_EVENT'
, I'd like to do it without the second arg:
emitter.emit('ANOTHER_EVENT') // ERROR, expected 2 arguments, but got 1.
which gives an error because it expects the second arg. (playground link)
要使其正常运行,我必须写:
To make it work, I have to write:
emitter.emit('ANOTHER_EVENT', undefined)
(仅在用'ANOTHER_EVENT'
调用emit
的情况下,如何才能使第二个arg不是必需的?
How can we make the second arg not required only for the case where we call emit
with 'ANOTHER_EVENT'
?
推荐答案
您可以在rest参数中使用元组,以基于第一个参数更改参数的数量(甚至是可选性).
You can use tuples in rest arguments to change the number of arguments (and even their optionality) based on the first argument.
type Events = {
SOME_EVENT: number
OTHER_EVENT: string
ANOTHER_EVENT: undefined
}
type EventArgs<EventTypes, K extends keyof EventTypes> = EventTypes[K] extends undefined ?[]:[EventTypes[K]]
interface EventEmitter<EventTypes> {
on<K extends keyof EventTypes>(s: K, listener: (...v: EventArgs<EventTypes,K>) => void);
emit<K extends keyof EventTypes>(s: K, ...v: EventArgs<EventTypes,K>);
}
declare const emitter: EventEmitter<Events>;
emitter.on('SOME_EVENT', (payload) => testNumber(payload));
emitter.on('OTHER_EVENT', (payload) => testString(payload));
function testNumber(value: number) {}
function testString(value: string) {}
emitter.emit('OTHER_EVENT', 'foo')
emitter.emit('ANOTHER_EVENT')
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