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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Middleware</h1><h2 id="what-is-middleware">What is middleware?</h2><ul><li>Anything that terminates the <code>req, res</code> cycle counts as middleware. It is basically anything that acts as an intermediary once the request is received but before the resource is sent. A good example would be the <code>app.use(express.json()</code> or <code>app.use(bodyParser.json)</code> functions we call in order to be able to parse JSON that is sent from the client.</li><li>You will most likely have multiple middleware functions running at once. We call this intermediary part of the cycle the <strong>request processing pipeline</strong>.</li><li>Generally all middleware will be added as a property on the Express <code>app</code> instance with the <code>app.use(...)</code> syntax.</li></ul><h2 id="creating-custom-middleware-functions">Creating custom middleware functions</h2><h3 id="basic-schema">Basic schema</h3><pre><code class="js language-js">app.use((req, res, next) => {
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// do some middleware
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next();
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});</code></pre><h3 id="next"><code>next</code></h3><p>The <code>next</code> parameter is key, it allows Express to move onto the next middleware function once the custom middleware executes. Without it, the request processing pipeline will get blocked.</p><p>Middleware functions are basically asynchronous requests and as such they use a similar syntax as Promises (e.g <code>then</code>) for sequencing processes.</p><h3 id="example-of-sequence">Example of sequence</h3><pre><code class="js language-js">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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});</code></pre><blockquote><p>It makes more sense of course to define our middleware within a function and then pass it as an argument to <code>app.use()</code></p></blockquote><h2 id="including-middleware-based-on-environment">Including middleware based on environment</h2><p>With a full-scale Node application you will typically run three environments:</p><ul><li>Development</li><li>Testing</li><li>Production</li></ul><p>We will not want to run certain types of middleware in all environments. For example, it would be costly to run logging in the app’s production environment. It would make more sense to run this only in development.</p><h3 id="accessing-current-node-environment">Accessing current Node environment</h3><p>We can control which middleware we run via the Node envrionment variables: <code>process.env</code> (see for instance <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Ports"><a href="Ports_in_NodeJS.html">ports</a></span></span>).</p><p>We could set <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Morgan"><a href="Morgan.html">Morgan</a></span></span> to run only in development with:</p><pre><code class="js language-js">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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}</code></pre></div></article><nav class="ui attached segment deemphasized backlinksPane" id="neuron-backlinks-pane"><h3 class="ui header">Backlinks</h3><ul class="backlinks"><li><span class="zettel-link-container cf"><span class="zettel-link"><a href="Using_GraphQL_with_Node.html">Using GraphQL with Node.js</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>Next we introduce GraphQL as a piece of Node.js <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Middleware"><a href="Middleware_in_NodeJS.html">middlewear</a></span></span>, with the <code>app.use()</code> method.</p></div></li></ul></li><li><span class="zettel-link-container cf"><span class="zettel-link"><a href="0_Introduction.html">Creating a RESTful API: Introduction</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>We first create an instance of Express within <code>index.js</code>. This will be the main coordinating file and we will aim to minimise the amount of business logic we have in this file. It should really just be for initialization and managing <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Middleware"><a href="Middleware_in_NodeJS.html">middleware</a></span></span>.</p></div></li></ul></li></ul></nav><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">middleware</span><span class="ui basic label zettel-tag" title="Tag">node-js</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html> |