TypeScript Type Assertion

Using as Keyword

The above Type Assertion can also be implemented by replacing the angular brackets with the as keyword,

For example

var num:any = 1111;
var assertedNum = num as number;
console.log("Type of variable assertedNum: "+ typeof(assertedNum));

In this code snippet, we converted the num to a number.

var num = "Hello";
var assertedNum = num as number;
console.log("Type of variable assertedNum: "+ typeof(assertedNum));

Type Assertion restricts the conversion of “Hello” to Number as it is not a feasible conversion.

Type Assertion with Objects

It often happens that there are objects declared without any properties. For this, the compiler will throw an error, which can be avoided using Assertion. For example,

let employee = {};
employee.Name = "Ninja"; 
employee.RegNo = 123;

Let us add Assertion using objects,

interface Employee {   
    Name: string;   
    RegNo: number;   
}  
let employee = <Employee> { };   
employee.Name = "Ninja"; 
employee.RegNo = 123; 
console.log(employee.Name);
console.log(employee.RegNo)

FAQs

1. Why is Type Assertion used?
   Type assertion allows you to set the value type and tell the compiler not to infer it. After Type Assertion, the programmer can get a better understanding of the variable types.
    
2. When to use Type Assertion?
   It is always advised to check the variable types beforehand so that there is no need for Assertion. Still, Assertion might be necessary in a few cases where Assertion is unavoidable. For instance, it is largely used to handle DOM events. For example, the document.getElementById returns HTMLElement. It can be cast to HTMLButtonElement using Assertion.
    
3. What are the different types of Type Assertion?
   We can implement Type Assertion in three ways, namely

• Assertion using angular(<>) brackets
• Assertion using “as” keyword
• Assertion using objects

Key Takeaways

This Blog covered all the necessary points about Assertion in Typescript, discussing in-depth its functionality and different methods of the appliance of Assertion. And also its implementation in Typescript.

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