Throwing errors in JavaScript

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javascript/js-throwing-errors.md
Sep 10, 2019
JavaScript
  • Throwing stops execution
  • Put the throw inside a try .. catch block
  • Once execution stops, the control will be passed to the first catch block it finds. If no catch then the program will terminate
  • You can re-throw errors
  • You can throw different types of errors
// throw Examples
throw 'Error2'; // generates an exception with a string value
throw 42;       // generates an exception with the value 42
throw true;     // generates an exception with the value true
throw new Error('Required');  // generates an error object with the message of Required

// Errors and Exceptions
console.log(e) // ReferenceError: blah is not defined
console.log(e.stack) // tells you line number, not supported by some
console.log(e.name) // ReferenceError
console.log(e.message) // blah is not defined

Example

```js /* * Complete the isPositive function. * If ‘a’ is positive, return “YES”. * If ‘a’ is 0, throw an Error with the message “Zero Error” * If ‘a’ is negative, throw an Error with the message “Negative Error” */ function isPositive(a) { // if (a === 0) { throw ‘Zero Error’} // if (a < 0) { throw ‘Negative Error’} // return ‘YES’

if (a >= 1) {
    return 'YES'
} else if (a === 0) {
    // put throw inside a try/catch so the execution doesn't stop
    try {
        throw new Error('Zero Error')
    } catch (e) {
        // e will be whatever error/exception we throwed earlier
        return e.message // 'Zero Error'
    }
} else {
    try {
        throw new Error('Negative Error')
    } catch (error) {
        return error.message
    }
}

} ````