问题
I'm reading through the code of the class-validator library and it has the following isInstance
method in it:
/**
* Checks if the value is an instance of the specified object.
*/
isInstance(object: any, targetTypeConstructor: new (...args: any[]) => any) {
return targetTypeConstructor
&& typeof targetTypeConstructor === "function"
&& object instanceof targetTypeConstructor;
}
Any thoughts on how to go about understanding the type new (...args: any[]) => any
? This is the first time I'm seeing this type of construct ...
回答1:
Let's reduce the type to smaller, bite size pieces that are more easily understood and then build back up to the full thing.
First, let's drop the new
from our mind and focus on the latter part of the definition:
(...args: any[]) => any
Next let's forget about the arguments for now:
() => any
Hopefully this is familiar as a function that returns type any
.
Next we can add back in the args:
(...args: any[]) => any
...args: any[]
is using the Rest Parameters construct which essentially says that there can be any number of parameters of the provided type any
. Because there are an unknown amount of any
parameters, the type of the argument is an array of any
.
So hopefully now it makes sense that this is a function that accepts any amount of arguments (of type any
) and returns type any
.
Finally we can add back the new
keyword to get:
new (...args: any[]) => any
The new
keyword here specifies that this function can be treated as a class constructor function and called with the new
keyword.
This gives us the whole picture that the function is a function that accepts any amount of arguments (of type any
) that returns type any
and can be used as a constructor function with the new
keyword.
When taken in the context of the API, it is essentially allowing you to pass any class constructor to the function.
回答2:
Breaking it down into pieces:
new
This keyword in TypeScript specifies what the constructor should look like, for the given property. Nice explanation here: https://stackoverflow.com/a/39623422/1678614.
(...args: any[]) => any
This syntax describe a function type (the constructor is a function).
...
is the ES6 spread operator. It's short hand for listing out all array elements one by one.
any[]
means that args
is an array and it's elements can be of any type.
=> any
specifies the return type of the function. In this case it allows the constructor to return any type.
回答3:
Means that the parameter targetTypeConstructor
is a function that accepts parameters and can be used as a constructor (you can use with new
keyword and create instances). You can pass a simple function or a class that are not abstracts.
For more you can check an example in the Typescript Playground
回答4:
new (...args: any[]) => any
This type specifies a function that accepts any number of arguments that are any
type, returns any
value, and can be invoked with new
.
A constructor is a special type of function that enforces invocation with the new
keyword at runtime, but in TypeScript, that can be statically detected, so targetTypeConstructor
is specifying an arbitrary constructor as the second parameter to isInstance()
.
To that end, it seems redundant to check
typeof targetTypeConstructor === "function"
as that's already enforced by targetTypeConstructor : new (...args: any[]) => any
at compile-time by TypeScript.
If the passed value is null
, it is already invalidated by the targetTypeConstructor && ...
conditional, which is necessary at runtime to prevent object instanceof targetTypeConstructor
from throwing TypeError
in the event that targetTypeConstructor
is actually null
due to Step 4
in §12.10.4 of the ECMAScript Specification:
12.10.4 Runtime Semantics: InstanceofOperator ( V, target )
- If Type(target) is not Object, throw a TypeError exception.
- Let instOfHandler be ? GetMethod(target, @@hasInstance).
If instOfHandler is not undefined, then
a. Return ToBoolean(? Call(instOfHandler, target, « V »)).
- If IsCallable(target) is false, throw a TypeError exception.
- Return ? OrdinaryHasInstance(target, V).
NOTE
Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If an object does not define or inherit @@hasInstance it uses the default instanceof semantics.
来源:https://stackoverflow.com/questions/50726326/how-to-go-about-understanding-the-type-new-args-any-any