How to handle errors from setTimeout in JavaScript?

Question

Simple question about try/catch for function in setTimeout

try {
    setTimeout(function () {
        throw new Error('error!');
    }, 300)
} catch (e) {
    console.log('eeee!')
    console.log(e)
}

Why doesn't catch block work?

What can I read about this?

P.S: the question is about possibility of handling errors like this. Don't answer about promises.

Answer 1

Functions scheduled to run with setTimeout are executed in the main loop, outside the body of code that originated them.

To handle errors, put the try-catch inside the setTimeout handler:

setTimeout(function () {
  try {
    throw new Error('error!');
  } catch (e) {
    console.error(e);
  }
}, 300)

If you need to access the Error object from block that called setTimeout, use Promises:

const promise = new Promise((resolve, reject) => {
  setTimeout(function () {
    try {
      throw new Error('error!');
      resolve(); // if the previous line didn't always throw

    } catch (e) {
      reject(e)
    }
  }, 300)
})

promise
  .then(result => console.log("Ok " + result))
  .catch(error => console.error("Ouch " + error))

This example above is not the most elegant way of handling the case with a Promise. Instead, implement a delay(ms) function like this:

function delay(ms) {
  return new Promise(resolve => setTimeout(resolve, ms))
}

Then call

delay(300).then(myFunction).catch(handleError)

Answer 2

You can find good explanation in this Node.js official doc.

The problem is that when the callback of your setTimeout() function executes the try { } catch(err) { } block is already exited. Also notice that the callback can crash Node.js process.

However if you want to handle the errors in the callback of setTimeout() function, then you can listen them by using process global EventEmitter object

process.on('uncaughtException', function(err){
  console.log(err)   
})

Answer 3

Because the catch block lexically surrounds the setTimeout call but that is not the function that throws. The direct translation, is

setTimeout(function () {
  try {
    throw new Error('error!');
  } catch (e) {
    console.log('eeee!');
    console.log(e);
  }
}, 300);

Answer 4

A bit strange solution, but sometimes it would be useful maybe...

function globalErrorHandler(e) {
  console.warn('eeee!')
  console.warn(e);
}

const _setTimeoutOriginal = setTimeout;
setTimeout = function(callback, timeout) {
  const args = Array.from(arguments).slice(2);
  _setTimeoutOriginal(function() {
    try {
      callback.apply(this, args);
    } catch (e) {
      globalErrorHandler(e);
    }
  }, timeout);
};

setTimeout(function() {
  throw new Error('error!');
}, 300)

Answer 5

When we have try/catch block, the code inside "try" block gets executed in that instant. When we have await syntax inside try/catch execution halts when the await line runs. In your code example, when setTimeout code gets executed, we are not in try/catch block anymore.

One way to handle is to use setTimeout from timers/promises. In test.mjs file, write this and execute with node test.mjs

import { setTimeout } from "timers/promises";

async function testing() {
  try {
    setTimeout(300);
    throw new Error("error!");
  } catch (e) {
    console.log("eeee!");
    console.log(e);
  }
}