34. ASYNC | Error Handling in JavaScript

The try...catch syntax allows us to catch errors so that the script instead of dying can do something more reasonable.

try {
  // try the code
} catch (err) {
  // handle the error
}

It works like this:

1. First, the code in try is executed.

2. If there is no error, catch is ignored else catch is executed.

The error object

For all the built-in errors, the error object has three main properties viz. 1) name, 2) message and 3) stack.

try {
  // try the code
} catch (err) {
  console.log(err.name);
  console.log(err.message);
  console.log(err.stack);
}

Throwing Custom Error

• We can throw our own error by using the throw syntax.

if (age>100) {
   throw new Error("Age can't be greater than 100.")
}

• We can also throw a particular error message by using the built-in constructor for standard errors.

throw new SyntaxError(message)

// OR

throw new ReferenceError(message)

finally block

It runs in all cases:

• after try if there are no errors.

• after catch if there are errors.

try {
  // try the code
} catch (err) {
  // handle the error
} finally {
  // runs in all cases
}

If a finally block has a return, it overrides any return from the try (or catch) block.

function test() {
  try {
    return "from try";
  } finally {
    return "from finally";
  }
}

console.log(test()); 

// Output: "from finally"

If finally has no return → original return is preserved.

function test() {
  try {
    return "from try";
  } finally {
    console.log("finally runs");
  }
}

console.log(test());

/* Output:
 finally runs
 from try
*/

Using return inside finally is generally discouraged because:

• It hides errors.

• It overrides expected return values.

• Makes debugging difficult.

try...catch works synchronously

If an error happens in scheduled code like Timers, etc., then try...catch won't catch it.

try {
  setTimeout(() => {
     // some error ocurred
  }, 2000)
} catch (err) {
  // handle the error
}

This is because the scheduled code is executed later when the JS engine has already left the try...catch construct.

Happy learning!