When does the catch block run?

JavaScriptJunior~2 min read

Short answer

The catch block runs only when an error (exception) occurs in the try block, either thrown manually (throw) or automatically (for example, ReferenceError, TypeError, and so on).

If the code inside try runs without errors, catch is skipped.

Step-by-step mechanism

The code inside try { ... } starts running line by line.
If it doesn't raise any errors, catch is not called.
If an error occurs inside try, execution of try stops immediately, and control moves to catch.
Inside catch(error) you have access to the error object, which you can handle or log.

Example 1. No error -> `catch` doesn't run

javascript

try {
  console.log("Everything is working great!");
} catch (error) {
  console.log("Caught an error:", error.message);
}

console.log("Continuing execution...");

Output:

javascript

Everything is working great!
Continuing execution...

Since there was no error inside try, catch doesn't run.

Example 2. There is an error -> `catch` runs

javascript

try {
  console.log("Start");
  throw new Error("An error occurred!");
  console.log("This code will not run");
} catch (error) {
  console.log("Caught:", error.message);
}

Output:

javascript

Start
Caught: An error occurred!

After throw, execution of try stops, and control passes to catch.

Example 3. An automatic error

javascript

try {
  console.log(a); // ReferenceError: a is not defined
} catch (error) {
  console.log("Caught:", error.name); // ReferenceError
}

Here catch runs because JS automatically threw an exception when accessing an undefined variable.

Example 4. Several lines - an error "cuts off" try

javascript

try {
  console.log("1");
  JSON.parse("not valid JSON"); // error here
  console.log("2"); // will not run
} catch (error) {
  console.log("Caught:", error.message);
}
console.log("3");

Output:

javascript

1
Caught: Unexpected token n in JSON at position 0
3

As soon as the error happens, try stops, and catch runs.

Example 5. An error outside `try` - `catch` doesn't run

javascript

try {
  console.log("Everything is fine inside try");
} catch (error) {
  console.log("Caught an error:", error.message);
}

throw new Error("Error outside try!"); // won't be caught

The error won't be caught, because it happened outside the try block.

Example 6. An error inside asynchronous code

Important: try...catch does not catch errors inside callbacks and promises unless you use await or .catch().

javascript

try {
  setTimeout(() => {
    throw new Error("Error in the callback"); // won't be caught
  }, 1000);
} catch (e) {
  console.log("Caught:", e.message);
}

To catch errors from promises, you need to use async/await or .catch():

Correct:

javascript

async function run() {
  try {
    await Promise.reject(new Error("Error in the promise"));
  } catch (e) {
    console.log("Caught:", e.message);
  }
}
run();

Example 7. If there are several errors, the first one is caught

javascript

try {
  throw new Error("Error 1");
  throw new Error("Error 2"); // will not run
} catch (err) {
  console.log("Caught:", err.message);
}

Output:

javascript

Caught: Error 1

Summary

Situation	Does catch run?	Reason
No errors in `try`	No	The code ran normally
`throw` in `try`	Yes	The exception was handled
JS itself threw an error (ReferenceError, etc.)	Yes	Automatic error
Error outside `try`	No	Not within the handled scope
Error in a callback/Promise without await	No	Asynchronous code outside try

Short Answer

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