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--- wiki2:nodejs:patterns [2017/05/11 20:48]
alfred
+++ wiki2:nodejs:patterns [2020/05/09 09:25] (actual)
@@ Línea 88: / Línea 88: @@
 });
 </code>
+Dependency injection is a software design pattern in which one or more dependencies (or services) are injected, or passed by reference, into a dependent object.
+Dependency injection is really helpful when it comes to testing. You can easily mock your modules' dependencies using this pattern.
+<code>
+var db = require('db');
+// do some init here, or connect
+db.init();
+var userModel = require('User')({
+  db: db
+});
+userModel.create(function (err, user) {
+});
+</code>
+<code>
+var test = require('tape');
+var userModel = require('User');
+test('it creates a user with id', function (t) {
+  var user = {
+    id: 1
+  };
+  var fakeDb = {
+    query: function (done) {
+      done(null, user);
+    }
+  }
+  userModel({
+    db: fakeDb
+  }).create(function (err, user) {
+    t.equal(user.id, 1, 'User id should match');
+    t.end();
+  })
+});
+</code>
+<code>
+function userModel (options) {
+  var db;
+  if (!options.db) {
+    throw new Error('Options.db is required');
+  }
+  db = options.db;
+  return {
+    create: function (done) {
+      db.query('INSERT ...', done);
+    }
+  }
+}
+module.exports = userModel;
+</code>
+In the example above we have two different dbs. In the index.js file we have the "real" db module, while in the second we simply create a fake one. This way we made it really easy to inject fake dependencies into the modules we want to test.
 ===== Middleware / Pipeline =====
@@ Línea 135: / Línea 196: @@
 </code>
-code>
+<code>
     // app.js
     require('./foo.js');

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