How do you create an error manually?

JavaScriptJunior~1 min read

Short answer

To create an error manually, write:

javascript
throw new Error("Error message");

How it works

new Error("text") - creates an error object.
throw - raises (generates) this exception.
JS interrupts execution of the current block and looks for the nearest try...catch.
If a handler is found, the error "lands" there. If not, the program crashes with the error message.

Example 1. Simple error creation

javascript

try {
  throw new Error("Something went wrong!");
} catch (err) {
  console.log("Caught:", err.message);
}

Output:

javascript

Caught: Something went wrong!

Example 2. Data validation

A common scenario - validating input parameters:

javascript

function divide(a, b) {
  if (typeof a !== "number" || typeof b !== "number") {
    throw new TypeError("Arguments must be numbers!");
  }
  if (b === 0) {
    throw new Error("Division by zero is not allowed!");
  }
  return a / b;
}

try {
  console.log(divide(10, 0));
} catch (e) {
  console.log("Error:", e.message);
}

Output:

javascript

Error: Division by zero is not allowed!

Example 3. Using different error types

You can throw not only Error, but also its descendants:

javascript

throw new TypeError("Wrong type");
throw new RangeError("Value out of range");
throw new ReferenceError("Undefined variable");

They are all created the same way, via new ...Error("message").

Example 4. Your own (custom) error

You can create your own error type by extending Error:

javascript

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

function validateUser(user) {
  if (!user.name) {
    throw new ValidationError("Name is required!");
  }
}

try {
  validateUser({});
} catch (e) {
  console.log(`${e.name}: ${e.message}`);
}

Output:

javascript

ValidationError: Name is required!

Example 5. You can throw things other than `Error` (but you shouldn't)

javascript

throw "An arbitrary string";
throw 404;
throw { message: "Error", code: 123 };

However this is not recommended - it breaks the standard error format and makes the code less predictable.

It's better to always use:

javascript

throw new Error("Message");

Example 6. An error without try...catch

If throw is used without handling, the program stops:

javascript

throw new Error("Critical error!");
console.log("This code will not run");

Output:

javascript

Uncaught Error: Critical error!

Summary

What we use	Purpose
`throw`	Raises an exception
`Error`	The base error class
`TypeError`, `RangeError`, `ReferenceError`	Specialized error types
`try...catch`	Handles the error
`class CustomError extends Error`	Creating your own errors

Short Answer

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