Error Handling in TypeScript

When something goes wrong while a program is running, TypeScript uses the try/catch paradigm for handling those errors. Try/catch is fairly common, Python uses a similar mechanism.

First, an Error Is Thrown

For example, let's say we try to access a property on an undefined variable. JavaScript will automatically "throw" an error.

const speed = car.speed;
// The code crashes with the following error:
// "ReferenceError: car is not defined"

Trying and Catching Errors

By wrapping that code in a try/catch block, we can handle the case where car is not yet defined. In TypeScript, the catch block parameter is typed as unknown by default, so you need to check if the error is an instance of Error before accessing its properties, like message. This ensures type safety and prevents runtime errors when the error is not an Error object.

try {
  const speed = car.speed;
} catch (err: unknown) {
  if (err instanceof Error) {
    console.log(`An error was thrown: ${err}`);
    // the code cleanly logs:
    // "An error was thrown: ReferenceError: car is not defined"
  } else {
    console.log("An unknown error occurred:", err);
  }
}

Handling a New Error Object

When handling a thrown Error object, you must access the message property of the error object to display it correctly to the console.

let err = new Error("This is the error message");

try {
  // computation
} catch (err) {
  if (err instanceof Error) {
    console.log(`An error was thrown: ${err.message}`);
    // the code cleanly logs:
    // "An error was thrown: This is the error message"
  }
}

Assignment

The following code is failing to execute. It is throwing an error string that isn't being handled gracefully. As a result, the entire program is crashing.

Properly handle the error in the code by wrapping all of the function calls within a single try...catch block. The catch block should log any error messages to the console, without extra formatting.

printUserExperience(4);

printUserExperience(10);

printUserExperience(200);

// don't touch below this line

function printUserExperience(level: number) {

if (level < 1 || level > 100) {

throw new Error("Experience level must be between 1 and 100!");

}

console.log(`The user has ${level} years of experience!`);

}