Summary - Strict mode

Write a safe account program by enabling strict options.

Summary - Safe account with strict mode

Turning on strict: true in an account program brings many protections at once, such as null safety and strict function types. An account may be null; we express this with Account | null and check for null in every operation.

interface Account { id: string; balance: number; }
function deposit(acc: Account | null, amount: number): Account {
  if (acc === null) {
    return { id: "yeni", balance: amount };
  }
  return { id: acc.id, balance: acc.balance + amount };
}

Exercise Safe account program

Define an Account interface: id: string and balance: number. Write a function deposit(acc: Account | null, amount: number): Account: if acc is null, return a new account (id: "yeni", balance: amount); otherwise update the balance with acc.balance + amount. Print two examples.

Show example solution

interface Account {
  id: string;
  balance: number;
}

function deposit(acc: Account | null, amount: number): Account {
  if (acc === null) {
    return { id: "yeni", balance: amount };
  }
  return { id: acc.id, balance: acc.balance + amount };
}

console.log(deposit(null, 100).balance);
console.log(deposit({ id: "a1", balance: 50 }, 30).balance);

Test yourself

Which of the following strict options provides null safety?
- alwaysStrict
- noImplicitAny
- strictNullChecks
- strictFunctionTypes
Show answer

strictNullChecks — strictNullChecks makes null and undefined separate from other types, providing null safety.

Nullable account handler

type AccountHandler = (acc: Account ____ ) => void;

Show answer

type AccountHandler = (acc: Account | null ) => void;

Strict mode account parts

strict: true

Enables all options at once
Account | null

An account type that can be null
acc === null

Narrowing via null check
acc.balance + amount

Balance update

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