Understanding the never Type in TypeScript

Discover the never type in TypeScript, which represents values that never occur. Learn about its use cases, including functions that throw errors or enter infinite loops, and how it helps in managing exceptional cases in your code.

TypeScript: The never Type

The never type in TypeScript represents values that never occur. It's primarily used in two scenarios:

Understanding never

Functions that Never Return:

• Functions that throw an error.
• Functions that enter an infinite loop.

function error(message: string): never {
    throw new Error(message);
}

function infiniteLoop(): never {
    while (true) {
        console.log('This loop never ends');
    }
}

never vs. void

While both never and void represent the absence of a return value, there's a crucial difference:

• void functions can implicitly return undefined.
• never functions guarantee that they will never return normally.

function myVoidFunction(): void {
    console.log('This function returns undefined implicitly');
}

let result1 = myVoidFunction(); // result1 is undefined

function myNeverFunction(): never {
    throw new Error('This function never returns');
}

let result2: never = myNeverFunction(); // Error: Type 'never' is not assignable to type 'never'

never in Type Narrowing

When using type guards, if all possible types are eliminated, the resulting type is never.

function isNumber(x: any): x is number {
    return typeof x === 'number';
}

function doSomething(x: any) {
    if (isNumber(x)) {
        // x is definitely a number here
    } else {
        // x is never here, as all possible types have been eliminated
        x; // Error: Type 'never' has no properties
    }
}

Key Points

• never represents values that never occur.
• Functions that throw errors or enter infinite loops have a return type of never.
• never is a subtype of all types, but no type is a subtype of never.
• void functions can implicitly return undefined, while never functions never return.

By understanding the never type, you can write more precise and informative TypeScript code.