Catching runtime errors in Javascript - Step-by-Step Execution

Concept Flow - Catching runtime errors

Start

↓

Try block runs

↓

Error occurs?

No→Try block ends normally

Yes↓

Catch block runs

↓

Finally block runs (optional)

↓

Program continues

The program tries to run code inside try. If an error happens, it jumps to catch. Finally runs always, then program continues.

Execution Sample

Javascript

try {
  let result = 10 / 0;
  console.log('Result:', result);
} catch (error) {
  console.log('Error caught:', error.message);
}

This code tries to divide 10 by 0 and logs the result or catches an error if it happens.

Execution Table

Step	Action	Evaluation	Result
1	Enter try block	Start executing code inside try	No error yet
2	Calculate 10 / 0	10 divided by 0	Infinity (no error in JS)
3	console.log('Result:', result)	Prints 'Result: Infinity'	Output: Result: Infinity
4	Try block ends	No error thrown	Catch block skipped
5	Program continues	No catch block run	Program ends normally

💡 No runtime error occurs because dividing by zero in JavaScript returns Infinity, so catch block is not executed.

Variable Tracker

Variable	Start	After Step 2	Final
result	undefined	Infinity	Infinity

Key Moments - 2 Insights

Why doesn't the catch block run when dividing by zero?

In JavaScript, dividing by zero does not cause a runtime error; it returns Infinity. So the try block finishes normally (see execution_table step 2 and 4).

What happens if an actual error occurs inside try?

If an error happens, execution jumps immediately to the catch block (not shown here but explained in concept_flow).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 2?

A Infinity

B 0

C Error

D undefined

💡 Hint

Check the 'Evaluation' and 'Result' columns in row 2 of the execution_table.

At which step does the catch block run in this example?

A Step 3

B Never

C Step 4

D Step 2

💡 Hint

Look at the 'Result' column in step 4; it says 'Catch block skipped'.

If the code inside try threw an error, what would happen?

A Try block continues after error

B Program crashes immediately

C Catch block runs to handle the error

D Finally block is skipped

💡 Hint

Refer to the concept_flow diagram showing error jumps from try to catch.

Concept Snapshot

try {
  // code that might cause error
} catch (error) {
  // code to handle error
}

- Code in try runs first
- If error occurs, catch runs
- No error means catch skipped
- Finally block can run always (optional)

Full Transcript

This example shows how JavaScript handles runtime errors using try and catch. The program tries to run code inside the try block. If an error happens, it jumps to the catch block to handle it. In this example, dividing 10 by 0 does not cause an error but returns Infinity, so the catch block does not run. The execution table shows each step: starting try, calculating division, printing result, and ending try without error. Variables like 'result' change from undefined to Infinity. Key points include understanding that not all errors throw exceptions (like division by zero in JS) and that catch only runs if an error occurs. The quiz asks about variable values, when catch runs, and what happens if an error occurs. The quick snapshot summarizes the try-catch syntax and behavior.