[英]Type annotation for `this` keyword in Typescript
I have a standalone function that's meant to use the context provided by Function.prototype.call
. 我有一个独立的函数,它意味着使用
Function.prototype.call
提供的上下文。
For example: 例如:
function foo () {
return this.bar;
}
> foo.call({bar: "baz"})
baz
Is there a way to provide a Typescript type annotation for the this
keyword in this scenario? 有没有办法在此场景中为
this
关键字提供Typescript类型注释?
It's a bit ugly, in my opinion. 在我看来,它有点难看。
First of all, you can use the special this
parameter syntax to identify the type of object you expect this
to be: 首先,你可以使用特殊
this
参数的语法来识别你所期望的对象类型this
是:
function foo (this: {bar: string}) {
return this.bar; // no more error
}
which helps if you call it directly: 如果直接调用它会有帮助:
foo(); // error, this is undefined, not {bar: string}
var barHaver = { bar: "hello", doFoo: foo };
barHaver.doFoo(); // acceptable, since barHaver.bar is a string
var carHaver = { car: "hello", doFoo: foo };
carHaver.doFoo(); // unacceptable, carHaver.bar is undefined
But you want to use foo.call()
. 但是你想使用
foo.call()
。 Unfortunately the Function.prototype.call()
typing in TypeScript won't really enforce this restriction for you: 不幸的是,在TypeScript中键入的
Function.prototype.call()
不会真正为您强制执行此限制:
foo.call({ bar: "baz" }); // okay, but
foo.call({ baz: "quux" }); // no error, too bad!
Merging something better into TypeScript's Function
declaration caused me problems, (First point of ugliness; you will need to cast foo
to something) so you can try something like this: 将更好的东西合并到TypeScript的
Function
声明中会给我带来问题,(丑陋的第一点;你需要将foo
为某些东西)所以你可以尝试这样的东西:
interface ThisFunction<T extends {} = {}, R extends any = any, A extends any = any> {
(this: T, ...args: A[]): R;
call(thisArg: T, ...args: A[]): R;
}
A ThisFunction<T,R,A>
is a function with a this
of type T
, a return value of type R
, and a rest argument of type A[]
. 甲
ThisFunction<T,R,A>
是具有一个功能this
类型的T
,类型的返回值R
和类型的其余参数A[]
(Second point of ugliness: you can't easily specify multiple arguments of different types in a way that will be enforced by the type system.) (第二点丑陋:你不能以类型系统强制执行的方式轻松指定不同类型的多个参数。)
You can then cast foo
to ThisFunction<{ bar: string }, string>
, (Third point of ugliness: the type system just will not infer this
types) and then finally use call()
: 然后你可以将
foo
为ThisFunction<{ bar: string }, string>
,(第三点丑陋:类型系统不会推断出this
类型)然后最后使用call()
:
(<ThisFunction<{ bar: string }, string>>foo).call({ bar: "baz" }); // okay, and
(<ThisFunction<{ bar: string }, string>>foo).call({ baz: "quux" }); // error, hooray!
Hope that helps! 希望有所帮助!
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