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:

1const customErrorMessage = "404"
2const ourError = new Error(customErrorMessage)

For example, we can use this Error like this:

1const everythingOK = false
2
3function showErrorMessage(customErrorMessage) {
4
5  const ourError = new Error(customErrorMessage)
6  console.log(ourError.message)
7  
8}
9
10if (everythingOK == false) {
11  showErrorMessage("something is wrong!")
12} 
13
14console.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:

1const everythingOK = false
2
3function showErrorMessage(customErrorMessage) {
4
5  const ourError = new Error(customErrorMessage)
6
7  // throwing instead of just printing
8  console.log(throw(ourError))
9  
10}
11
12if (everythingOK == false) {
13    showErrorMessage("not everything is OK!")
14}
15
16// this won't print if everythingOK is false
17console.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

1try {
2
3  // code that should work
4  if (codeOK) {
5      // do stuff
6  } else {
7      // if it fails, then show error
8      throw new Error("not working!")
9  }
10    
11} catch(e) {
12
13  // handle the error if it fails
14  console.log(e)    
15  // this would print "Error: not working!"
16
17  // further instructions 
18  
19}

The finally block

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

1try {
2
3  // code that should work
4  if (codeOK) {
5    // do stuff
6  } else {
7    // if it fails, then show error
8    throw new Error("not working!")
9  }
10    
11} catch(e) {
12
13  // handle the error if it fails
14  console.log(e)    
15  // this will print "Error: not working!"
16
17} finally {
18
19  // this will still print regardless
20  console.log("keep the party going!")
21
22}

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