strictNullChecks deep

Learn the deep behavior of strictNullChecks.

strictNullChecks deep

With strictNullChecks on, null and undefined cannot be assigned to other types. This affects property access, method calls, and return types. Calling .toUpperCase() on a string | null value is a compile error; a null check is required first.

function upper(v: string | null): string {
  if (v === null) return "";
  return v.toUpperCase();  // string
}

function bad(v: string | null): string {
  return v.toUpperCase();  // error: v may be null
}

Flow analysis

TypeScript performs flow analysis to determine whether a variable is null at a point. After a null check in an if condition, the variable is narrowed to non-null type in the subsequent code. This provides safe access.

function len(v: string | null): number {
  if (v !== null) {
    return v.length;  // string
  }
  return 0;
}

Exercise Handle nullable with strictNullChecks

Write an upper(v: string | null): string function. If v === null, return ""; otherwise return v.toUpperCase(). Print results via console.log(upper("hello")) and console.log(upper(null)).

Show example solution

function upper(v: string | null): string {
  if (v === null) {
    return "";
  }
  return v.toUpperCase();
}

console.log(upper("hello"));
console.log(upper(null));

Test yourself

What happens when a method is called directly on a value of type string | null with strictNullChecks on?
- The value becomes undefined
- Compile error: a null check is required first
- Runtime error
- The method is called automatically
Show answer

Compile error: a null check is required first — strictNullChecks considers the null possibility; calling .toUpperCase() on a string | null value is a compile error because null has no method.

Narrowing with null check

function len(v: string | null): number {
  if (v !== ____) {
    return v.length;
  }
  return 0;
}

Show answer

function len(v: string | null): number {
  if (v !== null) {
    return v.length;
  }
  return 0;
}

strictNullChecks deep parts

string | null

carries null possibility
if (v === null)

separates null branch
v.toUpperCase()

safe if not null
akış analizi

enables narrowing

Next lesson: Definite assignment

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