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This article introduces the relevant knowledge of "what are the main knowledge points of koa middleware". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Why the next function must and can only be called once in koa middleware? we can see that if next is not called, dispatch will not be triggered, and the next middleware will not be triggered, resulting in a timeout of the final request in the suspended state.

Koa middleware mainly involves several knowledge points of Promise:

The async function returns a Promise object [all middleware returns a promise object]

When the async function encounters an await call, it pauses the execution of the await function, waits for the result to be returned, and then continues to execute downwards.

An error within the async function will cause the returned Promise to become reject.

Now let's review a few questions raised earlier:

Why does the call to next () need to add await?

This is also the core of the onion ring invocation mechanism. When awaitnext () is executed, it executes next () [calls the next middleware] to wait for the result to be returned, and then executes downwards.

How to provide koa middleware with Context

Goal: encapsulate Context and provide convenient operation mode of request and response

/ / 1. Define KoaRequest, KoaResponse, KoaContext

InterfaceKoaContext {

Request?:KoaRequest

Response?:KoaResponse

Body:String | null

}

Constcontext:KoaContext= {

Getbody () {

Returnthis.response!.body

}

Setbody (body) {

This.response.bodykeeper body

}

}

FunctioncomposeMiddleware (middlewares:middlewareFn []) {

Return (context:KoaContext) = > {

Letstart=-1

Functiondispatch (i:number) {

/ /.. Omit other codes..

/ / 2. All middleware accept context parameters

Middleware (context, () = > {

Returndispatch (iTun1)

})

}

Returndispatch (0)

}

}

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