Middleware functions

 

1. Middleware functions

Middleware is a special kind of function in Express JS which accepts the request from

• the user (or)
• the previous middleware

After processing the request the middleware function

• sends the response to another middleware (or)
• calls the API Handler (or)
• sends response to the user

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app.method(Path, middleware1, handler);

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Example

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const jsonMiddleware = express.json();

app.use(jsonMiddleware);

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It is a built-in middleware function it recognizes the incoming request object as a JSON object, parses it, and then calls handler in every API call

1.1 Multiple Middleware functions

We can pass multiple middleware functions

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app.method(Path, middleware1, middleware2, handler);

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2. Logger Middleware Implementation

2.1 Defining a Middleware Function

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const middlewareFunction = (request, response, next) => {};

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2.2 Logger Middleware Function

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const logger = (request, response, next) => {

console.log(request.query);

next();

};

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The next parameter is a function passed by Express JS which, when invoked, executes the next succeeding function

2.3 Get Books API with Logger Middleware

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app.get("/books/", logger, async (request, response) => {

const getBooksQuery = `

SELECT

*

FROM

book

ORDER BY

book_id;`;

const booksArray = await db.all(getBooksQuery);

response.send(booksArray);

});

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3. Authenticate Token Middleware

In Authenticate Token Middleware we will verify the JWT Token

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const authenticateToken = (request, response, next) => {

let jwtToken;

const authHeader = request.headers["authorization"];

if (authHeader !== undefined) {

jwtToken = authHeader.split(" ")[1];

}

if (jwtToken === undefined) {

response.status(401);

response.send("Invalid JWT Token");

} else {

jwt.verify(jwtToken, "MY_SECRET_TOKEN", async (error, payload) => {

if (error) {

response.status(401);

response.send("Invalid JWT Token");

} else {

next();

}

});

}

};

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4. Get Books API with Authenticate Token Middleware

Let's Pass Authenticate Token Middleware to Get Books API

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app.get("/books/", authenticateToken, async (request, response) => {

const getBooksQuery = `

SELECT

*

FROM

book

ORDER BY

book_id;`;

const booksArray = await db.all(getBooksQuery);

response.send(booksArray);

});

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5. Passing data from Authenticate Token Middleware

We cannot directly pass data to the next handler, but we can send data through the request object

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const authenticateToken = (request, response, next) => {

let jwtToken;

const authHeader = request.headers["authorization"];

if (authHeader !== undefined) {

jwtToken = authHeader.split(" ")[1];

}

if (jwtToken === undefined) {

response.status(401);

response.send("Invalid JWT Token");

} else {

jwt.verify(jwtToken, "MY_SECRET_TOKEN", async (error, payload) => {

if (error) {

response.status(401);

response.send("Invalid JWT Token");

} else {

request.username = payload.username;

next();

}

});

}

};

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6. Get User Profile API with Authenticate Token Middleware

We can access request variable from the request object

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app.get("/profile/", authenticateToken, async (request, response) => {

let { username } = request;

const selectUserQuery = `SELECT * FROM user WHERE username = '${username}'`;

const userDetails = await db.get(selectUserQuery);

response.send(userDetails);

});

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