Default type parameter

Learn to give a default value to a type parameter.

Default type parameter

You can give a type parameter a default with =: <T = string>. If you omit the type argument at the call, T becomes the default; if you give one, that type is used. This is handy for helpers that usually want one common type but occasionally want another.

function depola<T = string>(x: T): T[] {
  return [x];
}
depola("metin");       // T = string
depola<number>(5);  // T = number

Default together with a bound

A default type can also be used together with a bound: <T extends Uzunluklu = string[]>. Here T must be at least Uzunluklu and defaults to string[]. The bound is a condition the default type must also satisfy; otherwise it is a compile error.

Exercise Single-item array with a default type

Write the generic function sarma<T = string>(x: T): T[] with body return [x];. Print the result of sarma("merhaba") with console.log.

Show example solution

function sarma<T = string>(x: T): T[] {
  return [x];
}
console.log(sarma("merhaba"));

Test yourself

In <code>depola<T = string>(x: T)</code>, what does the call <code>depola("merhaba")</code> do to T?
- Gives a compile error
- T becomes undefined
- T becomes any
- T becomes string (both default and inferred)
Show answer

T becomes string (both default and inferred) — Since no type argument is given, the default string applies; and since "merhaba" is a string, inference also yields string. The two agree.

Give a default type

function ilk<T = ____>(dizi: T[]): T {
  return dizi[0];
}
const s = ilk();  // T varsayılan

Show answer

function ilk<T = string>(dizi: T[]): T {
  return dizi[0];
}
const s = ilk();  // T varsayılan

Default type parameter parts

<T = string>

Default type declaration
depola("metin")

Default takes effect
depola<number>(5)

Given type overrides
<T extends Uzunluklu = string[]>

Bound and default together

Next lesson: Summary - Generic constraints

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