Errors

Errors in JavaScript are issues that occur during runtime, represented by the Error object. This object provides information about the error type and message and can be used to throw exceptions or handle them in a try-catch block.

When JavaScript throws an error, it throws an error object that consists of a name and a message property. The name is the general type of the error, and the message is a human-readable description of the specific error that happened.

Thrown errors are caught by the next outer catch block of a try...catch...finally statement. They can also be thrown intentionally by the throw statement.

The Error Object

The error object holds information about the exception that was thrown in its three properties:

• name Sets or returns an error name. (Type of Error)
• message Sets or returns an error message. (Description of specific instance.)
• stack Returns a string representing the point in the code where the error was instantiated.

The following types of error can be returned by the name property:

• “EvalError” An error has occurred in the eval() function (Note: Depreciated in newer versions of JavaScript)
• “RangeError” A number “out of range” has occurred
• “ReferenceError” An illegal reference has occurred
• “SyntaxError” A syntax error has occurred
• “TypeError” A type error has occurred
• “URIError” An error in encodeURI() has occurred

These are some example messages for various types of errors:

• RangeError
  • invalid array length
  • invalid date

let arr = new Array(-1); // RangeError: Invalid array length

• ReferenceError
  • “x” is not defined
  • assignment to undeclared variable “x”

console.log(x); // ReferenceError: x is not defined

• SyntaxError
  • “x” is a reserved identifier
  • a declaration in the head of a for-of loop can’t have an initializer

// SyntaxError: 'for-of' loop variable can't be initialized
try {
  for (let i = 0 of [1, 2, 3]) {
    console.log(i);
  }
} catch (err) {
  console.log(err.name);    // "SyntaxError"
  console.log(err.message); // "'for-of' loop variable can't have an initializer"
}

• TypeError
  • “x” is not a function
  • “x” is read-only

let num = 42;
num.toUpperCase(); // TypeError: num.toUpperCase is not a function

• URIError
  • malformed URI sequence

// This is an invalid percent encoding (notice '%E0%' is incomplete).
try {
  let badURI = decodeURIComponent('%E0%');
} catch (err) {
  console.log(err.name); // "URIError"
  console.log(err.message); // "malformed URI sequence"
}