Error types and custom errors — Error handling

Built-in error types

JavaScript defines several error types, all descendants of Error:

Type	When it appears
`Error`	Generic error
`TypeError`	A value is not of the expected type (e.g. calling something that is not a function)
`RangeError`	A value is outside the allowed range
`SyntaxError`	Malformed code (e.g. invalid JSON)
`ReferenceError`	Using a variable that does not exist

You can check the type with instanceof:

try {
  null.method();
} catch (e) {
  console.log(e instanceof TypeError); // true
}

Custom errors

For errors specific to your domain (for example, "insufficient balance"), create a class that extends Error:

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

throw new InsufficientBalanceError("You do not have enough balance");

This lets you tell your errors apart from the system ones and handle them specifically in the catch.

Examples

A custom error

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = "ValidationError";
  }
}
try {
  throw new ValidationError("Required field");
} catch (e) {
  console.log(e.name + ": " + e.message);
  console.log("Is it an Error?", e instanceof Error);
}