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src/js/service/userService.js

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+const Repository = require('../repository/repository');
+const passwordService = require('./passwordService');
+
+const userRepository = new Repository('users');
+
+class UserService {
+    /**
+     * Get a user from database by its login.
+     * @param {String} login User login.
+     * @param {Function} onSuccess Callback function to execute if a user exists with this login.
+     * @param {Function} onError Callback function to execute if not any user exists with this login.
+     */
+    getUser(login, onSuccess, onError) {
+        userRepository.find({login: login}, results => onSuccess(results[0]), onError);
+    }
+
+    /**
+     * Checks if credentials matches to an existing user, that have this password.
+     * @param {String} login User login.
+     * @param {String} password User password, in plain text.
+     * @param {Function} onSuccess Callback function to execute if a user exists with this login.
+     * @param {Function} onError Callback function to execute if not any user exists with this login.
+     */
+    checkCredentials(login, password, onSuccess, onError) {
+        this.getUser(
+            login,
+            dbUser => {
+                if (!!dbUser && passwordService.areSamePasswords(password, dbUser.password)) {
+                    onSuccess();
+                } else {
+                    onError();
+                }
+            },
+            // If login is incorrect, the "getUser" function will return "undefined".
+            // So if "user" is "undefined", this proofs that login is incorrect
+            onError
+        );
+    }
+}
+
+const singleton = new UserService();
+module.exports = singleton;