Never type and unreachable code in Typescript - Mini Project: Build & Apply

Never type and unreachable code

📖 Scenario: You are writing a TypeScript program that uses the never type to handle impossible cases. This helps catch errors early and avoid unreachable code.

🎯 Goal: Build a TypeScript function that uses the never type to handle unexpected input and demonstrate unreachable code detection.

📋 What You'll Learn

Create a function with a switch statement on a string parameter

Add a never type function to handle impossible cases

Use the never type function in the switch default case

Print the result of calling the function with valid and invalid inputs

💡 Why This Matters

🌍 Real World

Using the <code>never</code> type helps developers catch bugs early by ensuring all possible cases are handled, especially in complex decision logic.

💼 Career

Understanding unreachable code and the <code>never</code> type is important for writing robust TypeScript code in professional software development.

1

Create a function with a switch statement

Create a function called getColorCode that takes a parameter color of type string. Inside the function, write a switch statement on color with cases for 'red', 'green', and 'blue'. Return '#FF0000' for 'red', '#00FF00' for 'green', and '#0000FF' for 'blue'.

Typescript

function getColorCode(color: string): string {
    // Your code here
}

Need a hint?

Use a switch statement with case labels for each color and return the matching hex code string.

2

Add a never type function for impossible cases

Create a function called assertNever that takes a parameter value of type never and throws an Error with the message 'Unexpected value: ' + value.

Typescript

function getColorCode(color: string): string {
    switch (color) {
        case 'red':
            return '#FF0000';
        case 'green':
            return '#00FF00';
        case 'blue':
            return '#0000FF';
        default:
            throw new Error('Invalid color');
    }
}

// Your code here

Need a hint?

The assertNever function should accept only never type and throw an error with the unexpected value.

3

Use the never type function in the switch default case

Modify the getColorCode function to add a default case in the switch statement. In the default case, call assertNever(color as never) to handle unreachable code.

Typescript

function getColorCode(color: string): string {
    switch (color) {
        case 'red':
            return '#FF0000';
        case 'green':
            return '#00FF00';
        case 'blue':
            return '#0000FF';
        // Your code here
    }
}

function assertNever(value: never): never {
    throw new Error('Unexpected value: ' + value);
}

Need a hint?

Add a default case that calls assertNever with color as never to catch unreachable cases.

4

Print the result of calling the function

Write two console.log statements. The first calls getColorCode('red') and prints the result. The second calls getColorCode('yellow') inside a try block and catches the error to print 'Error caught: ' + error.message.

Typescript

function getColorCode(color: string): string {
    switch (color) {
        case 'red':
            return '#FF0000';
        case 'green':
            return '#00FF00';
        case 'blue':
            return '#0000FF';
        default:
            return assertNever(color as never);
    }
}

function assertNever(value: never): never {
    throw new Error('Unexpected value: ' + value);
}

// Your code here

Need a hint?

Use console.log to print the color code for 'red'. Use a try-catch block to catch and print the error message for 'yellow'.