TypeScript - Type Assertions

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Type assertion is a mechanism in TypeScript that informs the compiler of the variable type. We can override the type using a type assertion if TypeScript finds that the assignment is wrong. We must be certain that we are correct since the assignment is always legitimate when we employ a type assertion. If not, our program might not operate properly.

Type assertion functions similarly to typecasting, but unlike C# and Java, it does not do type verification or data rearrangement. Runtime support is provided for typecasting, although type assertion does not affect runtime. Type assertions are solely a compile-time construct to give the compiler indications about how we want our code to be inspected.

How to Perform Type Assertions?

Type assertion is a Typescript technique that tells the compiler about the type of variable. Though type assertion doesn’t recreate code, typecasting does. You can tell the compiler not to infer the type of a value by using type assertion. We utilize Type assertion to convert a variable from one type to another, such as any to a number. To do type assertion, we can either use the "<>" operator or the "as" operator. Typecasting provides runtime support, whereas type assertion has no impact on runtime. There are three techniques to perform Type Assertion in TypeScript, and those are:

• Using as operator
• Using <> operator
• Using object

Using as Operator for Type Assertion

The as keyword in TypeScript offers a method for displaying Type Assertion.

Syntax

let variable_any: any = 123
let variable_number: number = variable_any as number

In the above syntax, we used the "as" keyword on any type variable for type assertion.

Example

let variable_unknown: unknown = "Tutorialspoint";
console.log("variable_unknown value is: ", variable_unknown);
let variable_number: number = (variable_unknown as string).length;
console.log("Length of variable_unknown: ", variable_number);

On compiling, it will generate the following JavaScript code:

var variable_unknown = "Tutorialspoint";
console.log("variable_unknown value is: ", variable_unknown);
var variable_number = variable_unknown.length;
console.log("Length of variable_unknown: ", variable_number);

The output will be:

variable_unknown value is: Tutorialspoint
Length of variable_unknown: 14

Using <> Operator for Type Assertion

The <> operator is another way to perform type assertion in TypeScript.

Syntax

let variable_any: any = 123
let variable_number: number = <number> variable_any

In the above syntax, we used the "<>" operator on any type variable for type assertion.

Example

let my_number: unknown = 12345
console.log('my_number value is: ', my_number)
let num: number = <number>my_number
console.log('typeof num is: ', typeof num)

On compiling, it will generate the following JavaScript code:

var my_number = 12345;
console.log('my_number value is: ', my_number);
var num = my_number;
console.log('typeof num is: ', typeof num);

The output will be:

my_number value is: 12345
typeof num is: number

Using Object for Type Assertion

Objects are another way to perform the type assertion, unlike the "as" and "<>" operators; the objects can use for multiple type assertions at once.

Syntax

interface info {
   name: string,
   value: string
}
let my_obj = <info> { name: 'ABC', value: 'abc'}

In the above syntax, we used the object to perform type assertion.

Example

interface info {
   name: string
   value: string
}
let my_obj = <info>{}
my_obj.name = 'Tutorialspoint'
my_obj.value = 'typescript'
console.log(my_obj)

On compiling, it will generate the following JavaScript code:

var my_obj = {};
my_obj.name = 'Tutorialspoint';
my_obj.value = 'typescript';
console.log(my_obj);

The output will be:

{ name: 'Tutorialspoint', value: 'typescript' }