84 lines
2.2 KiB
Markdown
84 lines
2.2 KiB
Markdown
---
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tags:
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- node-js
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- middleware
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---
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# Middleware
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## What is middleware?
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- Anything that terminates the `req, res` cycle counts as middleware. It is
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basically anything that acts as an intermediary once the request is received
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but before the resource is sent. A good example would be the
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`app.use(express.json()` or `app.use(bodyParser.json)` functions we call in
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order to be able to parse JSON that is sent from the client.
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- You will most likely have multiple middleware functions running at once. We
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call this intermediary part of the cycle the **request processing pipeline**.
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- Generally all middleware will be added as a property on the Express `app`
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instance with the `app.use(...)` syntax.
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## Creating custom middleware functions
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### Basic schema
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```js
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app.use((req, res, next) => {
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// do some middleware
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next();
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});
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```
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### `next`
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The `next` parameter is key, it allows Express to move onto the next middleware
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function once the custom middleware executes. Without it, the request processing
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pipeline will get blocked.
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Middleware functions are basically asynchronous requests and as such they use a
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similar syntax as Promises (e.g `then`) for sequencing processes.
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### Example of sequence
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```js
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app.use((req, res, next) => {
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console.log("Do process A...");
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next();
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});
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app.use((req, res, next) => {
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console.log("Do process B...");
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next();
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});
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```
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> It makes more sense of course to define our middleware within a function and
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> then pass it as an argument to `app.use()`
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## Including middleware based on environment
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With a full-scale Node application you will typically run three environments:
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- Development
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- Testing
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- Production
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We will not want to run certain types of middleware in all environments. For
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example, it would be costly to run logging in the app's production environment.
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It would make more sense to run this only in development.
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### Accessing current Node environment
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We can control which middleware we run via the Node envrionment variables:
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`process.env` (see for instance [ports](Ports_in_NodeJS.md)).
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We could set
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[Morgan](Morgan.md) to run
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only in development with:
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```js
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if (app.get("env") === "development") {
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app.use(morgan("common"));
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console.log("Morgan enabled");
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}
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```
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