What does the throw operator do?

JavaScriptJunior~1 min read

Definition

throw is an operator that interrupts the execution of the current code and throws an exception (an error or any other object).

Syntax

javascript

throw expression;

Where expression can be any JS value: an object, a number, a string, a boolean, and so on. But most often an instance of the Error class is thrown.

Example 1. Simple usage

javascript

throw "Something went wrong!";

In the console:

javascript

Uncaught Something went wrong!

Here the program stops immediately, because the exception was not caught.

Example 2. With a try...catch block

javascript

try {
  throw new Error("An error occurred during execution!");
} catch (err) {
  console.log("Caught:", err.message);
}

What happens:

throw creates an exception - an Error object.
Execution of try is interrupted.
Control passes to catch.
The error is handled, the program continues.

Output:

javascript

Caught: An error occurred during execution!

Example 3. Using `throw` to validate input data

javascript

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

try {
  console.log(divide(5, 0));
} catch (e) {
  console.error("Error:", e.message);
}

Output:

javascript

Error: Division by zero is not possible!

Example 4. You can throw more than just Error

javascript

try {
  throw { message: "Network error", code: 500 };
} catch (e) {
  console.log(e.message, e.code);
}

Output:

javascript

Network error 500

But this is usually not recommended - it is better to always use the standard error classes (Error, TypeError, RangeError, and so on) to keep things readable and compatible.

Example 5. Code after throw does not run

javascript

console.log("before throw");
throw new Error("error!");
console.log("after throw"); // will not run

After throw, execution of the current block stops, and control passes to an error handler (if one exists).

Visually

javascript

try {
  // throw an exception
  throw new Error("Error!");
} catch (e) {
  // catch the exception
  console.log("Caught:", e.message);
} finally {
  console.log("This block always runs");
}

Summary

Point	Description
Purpose	Forcibly throws an exception (an error)
Value type	Any data type can be thrown (usually `Error`)
Effect	Interrupts execution of the current block
Used with	`try...catch` to handle errors
After throw	The code that follows does not run

Short Answer

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