How to create conditional types in TypeScript ?

Conditional types in TypeScript enable defining types based on conditions, similar to conditional statements in code. They determine different types of values, making functions adaptable to various input types, and enhancing code flexibility and maintainability.

Syntax: We can create conditional types with the used of the ternary operator and extend in TypeScript.

Type1 extends Type2 ? for One value : for different value;

Here extend works as a compare function that checks is Type1 has properties of Type2 if yes then it jumps to the true branch else to the false branch.

Now let’s understand through the following examples.

Example 1: Basic Conditional Types

In this example,  we will create the Conditional types which take the values and check it has the property of number or not with extends. If it has the property of number conditional types assign the types to the caller of conditional type else it assigns never to types.

// Distributive property of Typescript
type return_dis<D>= D extends number ? D : never;

type show = number;

// Conditional types for number
type new_number = return_dis<show>;

let n1 : new_number = 88;
console.log(n1);

console.log(typeof n1)

Output:

88
number

 Example 2: Narrowing Functionality of Input Data

Now, we will create the conditional type by narrowing the functionality of input data. In this type of creating conditional types, we will be filtering out the specific input type which contains some value. Here, we use extended function and set of values for narrowing with conditional types. First we will create the conditional types which checks the property if it is has a number string or Boolean or not then if it has 

type Conditional<G> = G extends {typeof : number|string|Boolean} ?
                      G :"This is an error";

let n = 55;
type nu = Conditional<typeof n>;

let s = "hello world";
type Str = Conditional<s>;

let b = Boolean;
type Boo = Conditional<typeof b>;

let k = null;
type SecondCond = Conditional<typeof k>;

let l1: nu = 88;
console.log(l1);

let l2: Str = "Hello Geeks";
console.log(l2);

let l3: Boo = true;
console.log(l3);

let l: SecondCond = "This is an error";
console.log(l);

Output:

88
Hello Geeks
true
This is an error

Example 3: Using Conditional Types for Multiple Data Types

In this example first, we will create the conditional types for the number and string in one conditional type. After that, we use the same conditional types for both string and number.

// Distributive property of Typescript
type return_dis<D>= D extends number|string ? D : never;

type show = number|"Geek";

// Conditional types for number
type new_number = return_dis<show>;

let n1 : new_number = 88;
console.log(n1);

// Same type is used for string also
type new_Geek = return_dis<show>;

let G1 : new_Geek = "Hey Geeks";

console.log(G1)

Output:

88
Hey Geeks