TypeScript Narrowing typeof type guards

In this article, we are going to learn about Narrowing typeof type guards. TypeScript is a popular programming language used for building scalable and robust applications. In TypeScript, the typeof type guard allows you to narrow down a variable’s type based on the result of the typeof operator. This is particularly useful when dealing with primitive types like string, number, boolean, symbol, and undefined, and for checking if a variable is a function (function) or an object (object).

The following are the types that can be identified by typeof operator:

• string
• number
• bigint
• boolean
• symbol
• undefined
• object
• function

Syntax

if (typeof variable === 'type') {
    // Code to run when the variable
  // matches the specified type.
}

Where,

• typeof: It is the TypeScript operator used to check the type of a variable.
• variable: It is the variable whose type you want to check.
• ‘type’: It is a string literal representing the expected type you want to check against.

Example 1: You can use typeof to check if a variable is of type string.

Javascript

const p = 'GeeksforGeeks';
if (typeof p === 'string') {

    // Here, TypeScript knows that 'p' is a string.
    console.log(p.toUpperCase());
}

Output

GEEKSFORGEEKS

Example 2: You can use typeof to check if a variable is of type number.

Javascript

const age = 30;
if (typeof age === 'number') {

    // TypeScript knows that 'age' is a number.
    console.log(age * 2);
}

Output

60

Example 3: You can use typeof to check if a variable is of type boolean.

Javascript

const isTrue = true;
if (typeof isTrue === 'boolean') {

    // TypeScript knows that 'isTrue' is a boolean.
    console.log(!isTrue);
}

Output

false

Example 4: You can use typeof to check if a variable is of type symbol.

Javascript

const uniqueSymbol = Symbol('unique');
if (typeof uniqueSymbol === 'symbol') {

    // TypeScript knows that 'uniqueSymbol' is a symbol.
    console.log(uniqueSymbol.description);
}

Output

unique

Example 5: You can use typeof to check if a variable is undefined.

Javascript

let someValue;
if (typeof someValue === 'undefined') {

    // TypeScript knows that
    // 'someValue' is undefined.
    console.log('Value is undefined.');
}

Output

Value is undefined.

Example 6: This example checks the typeof Objects. It doesn’t distinguish between different object types. Instead, it only tells you that a variable is not one of the other primitive types. We might need to use other type guards like instanceof or property existence checks to narrow down object types.

Javascript

const person = {
    name: 'GeeksforGeeks',
    age: 30,
};

if (typeof person === 'object') {
    console.log('person is an object');

    // You can access properties of
    // 'person' safely inside this block
    console.log(`Name: ${person.name}, Age: ${person.age}`);
}

Output

person is an object
Name: GeeksforGeeks, Age: 30

Example 7: You can use typeof to check if a variable is a function.

Javascript

function greet() {
    console.log('Hello!');
}

if (typeof greet === 'function') {

    // TypeScript knows that 'greet' is a function.
    greet();
}

Output

Hello!