Tuple inference
- TypeScript course
- Advanced
- Lesson 2/57
- 4 min
Learn tuple inference in generics.
What is tuple inference?
Tuple inference is the inference of the tuple type from the arguments passed in a generic function call. In function f<T extends unknown[]>(...args: T): T, T is determined as the tuple of the ordered types of the parameters in the call. This provides type-safe tuple return with rest parameters.
function f<T extends unknown[]>(...args: T): T {
return args;
}
f(1, "a", true); // [number, string, boolean]Tuple return with rest parameters
When a function returns rest parameters as a tuple, the return type preserves the type order of the call arguments. Unlike a normal array return, this keeps the element types and order fixed. So the call f(1, "a") returns [number, string].
Practice Tuple returning function
Define a generic function named dondur: <T extends unknown[]>(...args: T): T. Its body should be return args. Assign the result of calling dondur(1, "a", true) to a variable named sonuc. Print sonuc[0] with console.log.
Starter code
// dondur adinda generic fonksiyon tanimla
// rest parametre ile T tuple olarak cikarilsin
// fonksiyon kendisini geri döndürsün
// cagriyi sonuc'a ata
// sonuc[0] yazdirSample solution
function dondur<T extends unknown[]>(...args: T): T {
return args;
}
const sonuc = dondur(1, "a", true);
console.log(sonuc[0]);Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In the call f(1, "a") to function f<T extends unknown[]>(...args: T): T, what is T?
- unknown[]
- [number, string] Correct answer
- (number | string)[]
- any[]
Explanation: With rest parameters, the ordered types of the call arguments are inferred as a tuple; so T becomes [number, string], not a union array.
2 Rest parameter declaration
function f<T extends unknown[]>(____ args: T): T {
return args;
}function f<T extends unknown[]>(... args: T): T {
return args;
}Explanation: Rest parameters are declared with the ... prefix; this collects a variable number of arguments into a single array.
3 Tuple inference concepts
| Term | Meaning |
|---|---|
...args |
Rest parameter |
T extends unknown[] |
Tuple constraint |
[number, string] |
Inferred tuple type |
inference |
Type inference |