Database Basics and JS (CRUD) Get Certified
Almost every application is built around four core operations: CREATE, READ, UPDATE, and DELETE — commonly called CRUD.
In this lesson we build a simple in-memory database using a plain JavaScript array and implement each CRUD operation as a clean, reusable function.
Setting Up an In-Memory Database
What this code does: Creates an empty array that will act as our database and a counter for unique IDs.
let users = [];
let nextId = 1;
console.log("Database initialized:", users);
console.log("Next ID will be:", nextId);CREATE – Adding a Record
What this code does: Creates a new user object with a unique ID and adds it to the database array.
let users = [];
let nextId = 1;
function createUser(data) {
const user = { id: nextId++, ...data };
users.push(user);
return user;
}
const michael = createUser({ name: "Michael", age: 28 });
const david = createUser({ name: "David", age: 32 });
console.log("Created users:");
console.log(users);READ – Finding a Single Record
What this code does: Uses find() to return the first matching record or undefined if nothing is found.
let users = [
{ id: 1, name: "Michael", age: 28 },
{ id: 2, name: "David", age: 32 }
];
function findUserById(id) {
return users.find(user => user.id === id);
}
console.log("User with id 1:", findUserById(1));
console.log("User with id 999:", findUserById(999));READ – Getting All Records
What this code does: Returns the entire collection of users from the database.
let users = [
{ id: 1, name: "Michael", age: 28 },
{ id: 2, name: "David", age: 32 },
{ id: 3, name: "Emily", age: 24 }
];
function getAllUsers() {
return users;
}
console.log("All users:", getAllUsers());UPDATE – Modifying a Record
What this code does: Finds a user by ID and merges the new values into the existing record using Object.assign().
let users = [
{ id: 1, name: "Michael", age: 28 },
{ id: 2, name: "David", age: 32 }
];
function findUserById(id) {
return users.find(user => user.id === id);
}
function updateUser(id, updates) {
const user = findUserById(id);
if (!user) return null;
Object.assign(user, updates);
return user;
}
console.log("Before update:", findUserById(1));
updateUser(1, { age: 29 });
console.log("After update:", findUserById(1));DELETE – Removing a Record
What this code does: Locates the user by ID and removes it from the array with splice().
let users = [
{ id: 1, name: "Michael", age: 28 },
{ id: 2, name: "David", age: 32 },
{ id: 3, name: "Emily", age: 24 }
];
function deleteUser(id) {
const index = users.findIndex(user => user.id === id);
if (index === -1) return false;
users.splice(index, 1);
return true;
}
console.log("Before delete:", users.length, "users");
deleteUser(1);
console.log("After delete:", users.length, "users");
console.log(users);Generating Unique IDs
What this code does: Uses crypto.randomUUID() to create globally unique IDs instead of a simple counter.
function createUser(data) {
return {
id: crypto.randomUUID(),
...data
};
}
const user1 = createUser({ name: "Michael" });
const user2 = createUser({ name: "David" });
console.log("User 1 ID:", user1.id);
console.log("User 2 ID:", user2.id);
console.log("IDs are unique:", user1.id !== user2.id);Validating Data Before Creating
What this code does: Checks that a valid name is provided before creating a user. Throws a clear error if validation fails.
function createUser(data) {
if (!data.name || typeof data.name !== "string") {
throw new Error("A valid name is required");
}
return {
id: crypto.randomUUID(),
...data
};
}
try {
createUser({ age: 30 }); // missing name
} catch (error) {
console.log("Validation error:", error.message);
}
const validUser = createUser({ name: "Michael", age: 30 });
console.log("Successfully created:", validUser);Filtering and Querying Records
What this code does: Accepts any condition function and returns all matching records — similar to a SQL WHERE clause.
let users = [
{ id: 1, name: "Michael", age: 28 },
{ id: 2, name: "David", age: 35 },
{ id: 3, name: "Emily", age: 22 }
];
function findUsersWhere(predicate) {
return users.filter(predicate);
}
const adultsOver25 = findUsersWhere(user => user.age > 25);
console.log("Users older than 25:", adultsOver25);
const youngUsers = findUsersWhere(user => user.age < 25);
console.log("Users under 25:", youngUsers);Practice: Complete Mini CRUD App
What this code does: Combines create, read, update, and delete into one working example with validation.
let users = [];
function createUser(data) {
if (!data.name || typeof data.name !== "string") {
throw new Error("A valid name is required");
}
const user = { id: crypto.randomUUID(), ...data };
users.push(user);
return user;
}
function findUserById(id) {
return users.find(user => user.id === id);
}
function updateUser(id, updates) {
const user = findUserById(id);
if (!user) return null;
Object.assign(user, updates);
return user;
}
function deleteUser(id) {
const index = users.findIndex(user => user.id === id);
if (index === -1) return false;
users.splice(index, 1);
return true;
}
// Demo
const michael = createUser({ name: "Michael", age: 28 });
console.log("Created:", michael);
updateUser(michael.id, { age: 29 });
console.log("Updated:", findUserById(michael.id));
deleteUser(michael.id);
console.log("Remaining users:", users.length);Quick Summary
- CREATE →
push()a new object with a unique ID - READ →
find()for one record, return the whole array for all - UPDATE → find the record and use
Object.assign() - DELETE → find the index and use
splice() - Always validate data and handle “not found” cases cleanly
Tip: These same four operations appear in every real database — IndexedDB, SQLite, MongoDB, PostgreSQL. Master them here and the concepts transfer everywhere.
🧪 Test Your JavaScript Code
Edit the code on the left and click "Run Code" to see the console output.
Click "Run Code" to see the output here.