TypeScript Assertions Type

Last Updated : 11 Mar, 2025

TypeScript Assertions Type, also known as Type Assertion, is a feature that lets developers manually override the inferred or expected type of a value, providing more control over type checking in situations where TypeScript's automatic type inference may not be sufficient.

Syntax

let variableName: AssertedType = value as AssertedType;

Parameters

variableName: This is the name of the variable you want to assign the asserted value to.
AssertedType: This is the type you are telling TypeScript to treat the value as. It should be a valid TypeScript type.
value: This is the value that you want to assert the type for. It can be a variable, an expression, or a function return value.

Example 1: In this example, the value is initially declared as any(value as string), which is a type assertion, telling TypeScript to treat the value as a string. This allows us to access the length property on the assumed string type. We log both the original value and the length to the console.

JavaScript

let value: any = "Hello, Geek!"; let len: number = (value as string).length;  console.log(`Value: ${value}`); console.log(`Length: ${len}`);

Output

Value: Hello, Geek!
Length: 12

Example 2: In this example, defines an object 'person' with mixed types, then uses a type assertion to specify a more specific type for 'personDetails' and displays its 'name' and 'website' properties.

JavaScript

// Define an object with mixed types const person: any = {     name: "GeeksforGeeks",     website: "www.geeksforgeeks.org", };  // Type assertion to specify // a more specific type const personDetails = person as {     name: string;     website: string; };  // Access and display the person's details console.log(`Name: ${personDetails.name}`); console.log(`Website: ${personDetails.website}`);

Output

Name: GeeksforGeeks
website: www.geeksforgeeks.org

TypeScript Conditional Types

akshitsaxenaa09

Improve

Article Tags :

Similar Reads

Explain Type assertions in TypeScript

In TypeScript, type assertions allow developers to override the compiler's inferred type, informing it of the specific type of a value. Type assertions are purely compile-time constructs and do not alter the runtime behavior of the code. They are particularly useful when interfacing with APIs or thi

4 min read

Data types in TypeScript

In TypeScript, a data type defines the kind of values a variable can hold, ensuring type safety and enhancing code clarity. Primitive Types: Basic types like number, string, boolean, null, undefined, and symbol.Object Types: Complex structures including arrays, classes, interfaces, and functions.Pri

3 min read

TypeScript Assertion functions

TypeScript Assertion functions are nothing but the functions that allow us to mainly create the custom type guards that assert the type of a value on our specific criteria. The most suitable use of Assertion Functions is when we are working with the more complex data structures. Syntax:function asse

3 min read

TypeScript Conditional Types

In TypeScript, conditional types enable developers to create types that depend on a condition, allowing for more dynamic and flexible type definitions. They follow the syntax T extends U ? X : Y, meaning if type T is assignable to type U, the type resolves to X; otherwise, it resolves to Y.Condition

4 min read

TypeScript Custom Type Guards

TypeScript boasts a powerful type system, which helps the developers catch errors at compile time and write more resilient code. Sometimes you would need to work with complex types or dynamically typed data where the type information might not be readily available. In situations like these, TypeScri

7 min read

TypeScript Truthiness Narrowing Type

In this article, we are going to learn about Truthiness narrowing Type in Typescript. TypeScript is a popular programming language used for building scalable and robust applications. In TypeScript, truthiness narrowing is a concept that allows you to narrow down the type of a variable based on its t

3 min read

TypeScript Defining a Union Type

In this article, we are going to learn about Defining a Union Type in Typescript. TypeScript is a popular programming language used for building scalable and robust applications. In TypeScript, a union type allows a variable to have one of several possible types. You can define a union type by using

3 min read

What is Type Annotations in TypeScript ?

TypeScript Type Annotations allow developers to explicitly define the types of variables, functions, and other elements in the program. They help catch mistakes early by ensuring the right data type is used.They make the code easier to understand and follow.Type Annotations help avoid errors and mak

3 min read

Recursive Type Guards In TypeScript

In TypeScript type guards help determine the type of a variable at runtime, they are especially useful when dealing with complex types like unions, discriminated unions or even recursive structures and a recursive type guard is a type guard function that can handle complex nested types, including th

6 min read

Assertions in Cypress

Cypress is an open-source website testing tool that is used to automate tests for JavaScript web applications. It is designed for end-to-end testing and it can be used for unit tests and integration tests as well. It is fast, reliable, and can run in real-time directly in the browser. It's built to

5 min read