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Typescript Type Inference

In TypeScript, type inference is the process by which TypeScript automatically determines the type of a variable, parameter, or return value without explicit type annotations.

1. Variable Type Inference

When you declare a variable without specifying a type, TypeScript infers it based on the initial value:

let age = 25; // Inferred as `number`

In this case, age is inferred as a number because it’s assigned a numeric value. Once the type is inferred, you can’t assign values of a different type.

2. Function Return Type Inference

For functions, if the return type is not specified, TypeScript infers it based on the returned value(s):

function getGreeting(name: string) {
return `Hello, ${name}!`;
// Return type inferred as `string`
}

The function getGreeting is inferred to return a string because that’s the type of the value returned.

3. Contextual Typing

TypeScript can infer types based on the surrounding context, especially in scenarios like event handling or function callbacks:

document.addEventListener('click', (event) => {
console.log(event.target); // `event` inferred as `MouseEvent`
});

Here, event is inferred as MouseEvent because click events are associated with that type.

4. Best Common Type Inference

When working with arrays of mixed types, TypeScript infers the type as the most common type among the elements:

let mixedArray = [1, 2, null]; // Inferred as `(number | null)[]`

In this example, TypeScript infers the type as (number | null)[] because the array contains number and null values.

5. Inference in Generics

TypeScript can infer types for generic functions based on the provided arguments:

function identity<T>(value: T): T {
return value;
}
let output = identity("Hello"); // Inferred as `string`

Here, T is inferred as string because the argument passed is of type string

Summary

Type inference makes TypeScript more flexible by letting the compiler determine types automatically, reducing the need for redundant type annotations. Understanding TypeScript’s inference helps you write cleaner and more concise code. However, when inference isn’t sufficient or leads to ambiguity, it’s best to use explicit type annotations.