TypeORM for TypeScript: Complete Guide with Examples

What is TypeORM and why is it needed?

TypeORM is a powerful Object-Relational Mapping (ORM) library for TypeScript and JavaScript that works with various databases: PostgreSQL, MySQL, MariaDB, SQLite, MS SQL Server, Oracle, and MongoDB (partially). It provides a convenient way to interact with relational databases through an object-oriented approach, allowing developers to work with tables as classes and records as objects.

Main reasons to use TypeORM:

• Typing: Full integration with TypeScript provides strict typing and autocompletion in the IDE.
• Declarative approach: Defining the database schema through decorators or schemas.
• Auto-migrations: Automatic creation and synchronization of the database schema.
• Active Record and Data Mapper: Support for both popular patterns.
• Relation support: One-to-One, One-to-Many, Many-to-Many.
• Query Builder: Flexible query builder for complex SQL queries.
• Cross-platform: Works with Node.js, browsers, React Native, and Ionic.

Installing TypeORM

To install TypeORM and the necessary database drivers, use npm (or yarn):

// Install TypeORMnpm install typeorm --save

// Install reflect-metadata (required for decorators)npm install reflect-metadata --save

// Install driver for PostgreSQLnpm install pg --save

// Install driver for MySQL/MariaDBnpm install mysql2 --save

// Install driver for SQLitenpm install sqlite3 --save

// Install driver for SQL Servernpm install mssql --save

// Install driver for MongoDB (experimental)npm install mongodb --save

After installation, add to tsconfig.json:

{  "compilerOptions": {    "emitDecoratorMetadata": true,    "experimentalDecorators": true  }}

Quick start with code example

Let's create a simple application for managing users using SQLite (requires no server installation).

1. Create a file data-source.ts for connection configuration:

import { DataSource } from "typeorm";import { User } from "./entity/User";

export const AppDataSource = new DataSource({    type: "sqlite",    database: "database.sqlite",    synchronize: true,    logging: true,    entities: [User],    subscribers: [],    migrations: [],});

// Initialize connectionAppDataSource.initialize()    .then(() => {        console.log("Data Source has been initialized!");    })    .catch((err) => {        console.error("Error during Data Source initialization:", err);    });

2. Create the User entity in the file entity/User.ts:

import { Entity, PrimaryGeneratedColumn, Column } from "typeorm";

@Entity()export class User {    @PrimaryGeneratedColumn()    id: number;

    @Column()    firstName: string;

    @Column()    lastName: string;

    @Column({ unique: true })    email: string;

    @Column({ default: true })    isActive: boolean;}

3. Create a file index.ts to demonstrate the functionality:

import "reflect-metadata";import { AppDataSource } from "./data-source";import { User } from "./entity/User";

async function main() {    await AppDataSource.initialize();

    // Create a new user    const user = new User();    user.firstName = "Ivan";    user.lastName = "Petrov";    user.email = "ivan@example.com";    user.isActive = true;

    // Save to the database    await AppDataSource.manager