throw expression;

expression

The expression to throw.

The throw statement is valid in all contexts where statements can be used. Its execution generates an exception that penetrates through the call stack. For more information on error bubbling and handling, see Control flow and error handling.

The throw keyword can be followed by any kind of expression, for example:

throw error; // Throws a previously defined value (e.g. within a catch block)
throw new Error("Required"); // Throws a new Error object

In practice, the exception you throw should always be an Error object or an instance of an Error subclass, such as RangeError. This is because code that catches the error may expect certain properties, such as message, to be present on the caught value. For example, web APIs typically throw DOMException instances, which inherit from Error.prototype.

The syntax forbids line terminators between the throw keyword and the expression to be thrown.

throw
new Error();

The code above is transformed by automatic semicolon insertion (ASI) into:

throw;
new Error();

This is invalid code, because unlike return, throw must be followed by an expression.

To avoid this problem (to prevent ASI), you could use parentheses:

throw (
  new Error()
);

This example defines a function that throws a TypeError if the input is not of the expected type.

function isNumeric(x) {
  return ["number", "bigint"].includes(typeof x);
}

function sum(...values) {
  if (!values.every(isNumeric)) {
    throw new TypeError("Can only add numbers");
  }
  return values.reduce((a, b) => a + b);
}

console.log(sum(1, 2, 3)); // 6
try {
  sum("1", "2");
} catch (e) {
  console.error(e); // TypeError: Can only add numbers
}

This example calls a callback-based async function, and throws an error if the callback receives an error.

readFile("foo.txt", (err, data) => {
  if (err) {
    throw err;
  }
  console.log(data);
});

Errors thrown this way are not catchable by the caller and will cause the program to crash unless (a) the readFile function itself catches the error, or (b) the program is running in a context that catches top-level errors. You can handle errors more naturally by using the Promise() constructor.

function readFilePromise(path) {
  return new Promise((resolve, reject) => {
    readFile(path, (err, data) => {
      if (err) {
        reject(err);
      }
      resolve(data);
    });
  });
}

try {
  const data = await readFilePromise("foo.txt");
  console.log(data);
} catch (err) {
  console.error(err);
}

• try...catch
• Error