72 lines
1.8 KiB
Markdown
72 lines
1.8 KiB
Markdown
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## Writing Middleware
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Koa middleware are simple functions which return a `GeneratorFunction`, and accept another. When
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the middleware is run by an "upstream" middleware, it must manually `yield` to the "downstream" middleware.
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For example if you wanted to track how long it takes for a request to propagate through Koa by adding an
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`X-Response-Time` header field the middleware would look like the following:
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```js
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function responseTime(next){
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return function *(){
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var start = new Date;
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yield next;
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var ms = new Date - start;
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this.set('X-Response-Time', ms + 'ms');
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}
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}
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app.use(responseTime);
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```
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If you're a front-end developer you can think any code before `yield next;` as the "capture" phase,
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while any code after is the "bubble" phase. Here's another way to write the same thing, inline:
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```js
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app.use(function(next){
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return function *(){
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var start = new Date;
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yield next;
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var ms = new Date - start;
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this.set('X-Response-Time', ms + 'ms');
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}
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});
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```
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Next we'll look at the best practices for creating Koa middleware.
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## Middleware Best Practices
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When creating public middleware it's useful to conform to the convention of
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wrapping the middleware in a function that accepts options, allowing users to
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extend functionality. Even if your middleware accepts _no_ options, this is still
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a good idea to keep things uniform.
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Here our contrived `logger` middleware accepts a `format` string for customization,
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and returns the middleware itself:
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```js
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function logger(format){
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format = format || ':method ":url"';
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return function(next){
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return function *(){
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var str = format
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.replace(':method', this.method)
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.replace(':url', this.url);
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console.log(str);
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yield next;
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}
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}
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}
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app.use(logger());
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app.use(logger(':method :url'));
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```
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