JavaScript try/catch blocks [Tech Interview Quiz]

Last updated Jan 6, 2023

• JS Interview Quiz
• JavaScript

try blocks in JavaScript help you execute dangerous code that can throw errors. You can catch those errors in the catch blocks and handle them gracefully.

Error handling is an important part of software development, so try/catch questions appear very frequently in JavaScript technical interviews.

Consider the following code snippet and describe what would happen when you try to run it.

Code snippet as a picture

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  let e1, e2;  
  try {  
    console.log(null.length);  
  } catch (e) {  
    e1 = e;  
  }  
  try {  
    console.log(undefined.length);  
  } catch (e) {  
    e2 = e;  
  }  
  
  console.log(e1.message.split(' ')[0] === e2.message.split(' ')[0]);

How will the try/catch blocks behave? What will be logged to the console? (think well, then click for the answer and explanation)

Explanation

So, we have 2 variables and 2 try/catch blocks that supposedly catch errors and put them into e1 and e2.

Then, the content of errors is analyzed, compared and the comparison result is logged to the screen.

First, let’s determine what’s inside of e1 and e2. To do that, we need to check the code in the try blocks. Both trying to get to null.length and undefined.length will throw an error as neither undefined nor null have the length property.

These errors will be caught in the catch blocks as e and then assigned to the variables e1 and e2.

The content of these errors will be a bit different. If we were to log e.message to the screen in the catch block, we would see the following:

1 2	Cannot read property 'length' of null Cannot read property 'length' of undefined

Then, .split(' ')[0] gives us the first words of these sentences which is Cannot in both cases. So ultimately, the program can be simplified to:

1	console.log('Cannot' === 'Cannot')

Answer

The expression in the console.log will be evaluated as true and logged to the screen