throw, try & catch

Give a failure a clear meaning

Lesson 18 of 20 · 11 min · Module 6: Design, validate & debug

Some functions cannot produce a meaningful result for invalid input. Throwing an error lets the caller decide how to handle that failure.

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Use a consistent failure contract
Handle type and range errors separately

The idea

Create an appropriate error and throw it. A surrounding try/catch can recover. Catch only at a point where you can make a useful decision.

try {
  throw new RangeError("Score is outside 0–100");
} catch (error) {
  console.log(error.message);
}

01Describe the happy path

A valid value should pass through unchanged. Inclusive boundaries mean zero and one hundred are acceptable, rather than exceptional cases.

02Choose the error by the failed rule

A numeric-looking string violates the input type contract. The finite number 101 has the right type but violates the range contract.

03Let callers recover deliberately

A UI might catch the error and show a useful message. A calculation function should not silently replace a bad score with zero, because that disguises failure as data.

Your turn

Write requireScore(value). Return a finite number between 0 and 100 inclusive. Throw a TypeError for anything that is not a finite number, and a RangeError for a finite number outside that interval.

What the checks look for

Accepts both inclusive boundaries
Throws TypeError for nonnumeric or nonfinite values
Throws RangeError outside the interval

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Take this with you

A useful error explains which promise the input broke.

Go a little further

Write a caller that catches the error and returns {ok:false, message} for display.