strictNullChecks deep
- TypeScript course
- Advanced
- Lesson 45/57
- 4 min
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;
}Practice 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)).
Starter code
// upper fonksiyonunu nullable string parametreyle yaz
// null ise boş dize, değilse toUpperCase döndür
// upper("hello") ve upper(null) sonuçlarını yazdırSample solution
function upper(v: string | null): string {
if (v === null) {
return "";
}
return v.toUpperCase();
}
console.log(upper("hello"));
console.log(upper(null));Expected output
HELLO
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- Runtime error
- The method is called automatically
Explanation: strictNullChecks considers the null possibility; calling .toUpperCase() on a string | null value is a compile error because null has no method.
2 Narrowing with null check
function len(v: string | null): number {
if (v !== ____) {
return v.length;
}
return 0;
}function len(v: string | null): number {
if (v !== null) {
return v.length;
}
return 0;
}Explanation: The v !== null check narrows v to string in the condition body; this provides safe access under strictNullChecks.
3 strictNullChecks deep parts
| Term | Meaning |
|---|---|
string | null |
carries null possibility |
if (v === null) |
separates null branch |
v.toUpperCase() |
safe if not null |
akış analizi |
enables narrowing |