Error Handling

throw operator

standard errors

// built-in constructors for standard errors
let error = new Error(message);
let error = new SyntaxError(message);
let error = new ReferenceError(message);
// ...

syntax

let json = '{ "age": 30 }'; // incomplete data

try {
  let user = JSON.parse(json); // <-- no errors
  if (!user.name) {
    throw new SyntaxError("Incomplete data: no name"); // (*)
  }
} catch(e) {
  alert( "JSON Error: " + e.message ); // JSON Error: Incomplete data: no name
}

Rethrowing

• catch should only process errors that it knows and “rethrow” all others (use instanceof)

let json = '{ "age": 30 }'; // incomplete data

try {
  let user = JSON.parse(json);
  if (!user.name) {
    throw new SyntaxError("Incomplete data: no name");
  }
  blabla();           // unexpected error
} catch(e) {
  if (e instanceof SyntaxError) { 
    alert( "JSON Error: " + e.message );
  } else {
    throw e;          // rethrow (*)
  }
}

try...catch...finally

• finally is executed even with an explicit return.
• finally is executed just before returns to the outer code.

try {
   ... try to execute the code ...
} catch(e) {
   ... handle errors ...
} finally {
   ... execute always ...
}

try...finally

• Don’t want to handle errors.
• But want to be sure that processes are finalized.

custom errors, extending Error

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = "ValidationError";
  }
}

Wrapping exceptions

• a function handles low-level exceptions (PropertyRequiredError("age")), and creates higher-level errors (ReadError) instead of various low-level ones.