JavaScript error handling

A script will "error out" in two different ways if "something goes wrong":

• silently failing
  • the script continues to run but would yield unexpected or trivial output
• error handling
  • the script stops running and explicitly explains what has happened

We will review the latter way with these essential topics in error handling:

• Error object
• throw keyword
• try and catch blocks
  • finally block

The Error object

An Error is a class of objects; to create (or construct) an instance of one, we use the new keyword:

const customErrorMessage = "404"
const ourError = new Error(customErrorMessage)

For example, we can use this Error like this:

const everythingOK = false

function showErrorMessage(customErrorMessage) {

  const ourError = new Error(customErrorMessage)
  console.log(ourError.message)
  
}

if (everythingOK == false) {
  showErrorMessage("something is wrong!")
} 

console.log("The show goes on!")

However, creating this Error object does not cause the program to stop running; in which case, we would need to throw the error!

The throw keyword

Using the throw keyword stops the program at that point. Any further lines of code will not execute:

const everythingOK = false

function showErrorMessage(customErrorMessage) {

  const ourError = new Error(customErrorMessage)

  // throwing instead of just printing
  console.log(throw(ourError))
  
}

if (everythingOK == false) {
    showErrorMessage("not everything is OK!")
}

// this won't print if everythingOK is false
console.log("Everything is OK!")

The try and catch blocks

We can use try and catch blocks to handle errors in a structure similar to if and else:

• try would attempt to run a piece of code successfully
• catch(error) would contain the error handling should something go wrong
  • this block would contain the Error object

try {

  // code that should work
  if (codeOK) {
      // do stuff
  } else {
      // if it fails, then show error
      throw new Error("not working!")
  }
    
} catch(e) {

  // handle the error if it fails
  console.log(e)    
  // this would print "Error: not working!"

  // further instructions 
  
}

The finally block

If we still want to run code after the error gets thrown, we can use a finally block:

try {

  // code that should work
  if (codeOK) {
    // do stuff
  } else {
    // if it fails, then show error
    throw new Error("not working!")
  }
    
} catch(e) {

  // handle the error if it fails
  console.log(e)    
  // this will print "Error: not working!"

} finally {

  // this will still print regardless
  console.log("keep the party going!")

}

Note that a try block must always exist if a catch or finally exists; the following combination of blocks are possible:

• try and catch
• try and finally
• try and catch and finally

We should not see a combination with

• only one of the blocks
• catch and finally, but no try