Summary - Strict null

Write a safe configuration loader with strict null.

Summary - Strict null

Strict null, definite assignment, and non-null assertion together form a safe configuration loader. strictNullChecks catches null values; definite assignment uses ! for late-starting assignments; non-null assertion narrows with ! where the value is known not to be null.

class Config {
  host!: string;
  port!: number;

  load(env: { HOST: string | null; PORT: string | null }): void {
    this.host = env.HOST!;
    this.port = Number(env.PORT!);
  }
}

Exercise Write a configuration loader

Define a Config class: with host!: string and port!: number definite assignment properties. Write a load(env: { HOST: string | null; PORT: string | null }): void method: assign this.host = env.HOST! and this.port = Number(env.PORT!). Create a Config instance, call c.load({ HOST: "localhost", PORT: "8080" }), then print results via console.log(c.host) and console.log(c.port).

Show example solution

class Config {
  host!: string;
  port!: number;

  load(env: { HOST: string | null; PORT: string | null }): void {
    this.host = env.HOST!;
    this.port = Number(env.PORT!);
  }
}

const c = new Config();
c.load({ HOST: "localhost", PORT: "8080" });
console.log(c.host);
console.log(c.port);

Test yourself

What is the difference between definite assignment (!) and non-null assertion (!)?
- Non-null assertion performs a runtime check
- They are the same
- Definite assignment is used in property declaration, non-null assertion in expression
- Definite assignment is used only in classes
Show answer

Definite assignment is used in property declaration, non-null assertion in expression — Definite assignment uses ! in a class property declaration to say the property will be assigned later; non-null assertion uses ! after an expression to assert the value is not null.

Assign Config host

class Config {
  host!: string;
  port!: number;

  load(env: { HOST: string | null; PORT: string | null }): void {
    this.host = ____;
    this.port = Number(env.PORT!);
  }
}

Show answer

class Config {
  host!: string;
  port!: number;

  load(env: { HOST: string | null; PORT: string | null }): void {
    this.host = env.HOST!;
    this.port = Number(env.PORT!);
  }
}

Strict null summary matching

strictNullChecks

separates null and undefined from other types
definite assignment (!)

in property declaration
non-null assertion (!)

in expression
akış analizi

narrowing in if condition

Next lesson: AST model

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