Throwing errors in Javascript - Step-by-Step Execution

Concept Flow - Throwing errors

Start

↓

Check condition

Yes↓

Throw error

↓

Catch error?

No→Program stops

Yes↓

Handle error

↓

Continue or Exit

The program checks a condition, throws an error if needed, then either catches and handles it or stops.

Execution Sample

Javascript

function checkAge(age) {
  if (age < 18) {
    throw new Error('Too young');
  }
  return 'Allowed';
}

checkAge(15);

This code throws an error if age is less than 18, otherwise returns 'Allowed'.

Execution Table

Step	Action	Condition	Result	Output/Error
1	Call checkAge(15)	age = 15	Check if 15 < 18	No output yet
2	Condition true	15 < 18	Throw new Error('Too young')	Error thrown: Too young
3	No catch block	Error not caught	Program stops	Uncaught Error: Too young

💡 Error thrown at step 2 and not caught, so program stops with uncaught error.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
age	undefined	15	15	15

Key Moments - 2 Insights

Why does the program stop after throwing the error?

Because there is no catch block to handle the error, as shown in execution_table step 3, the error is uncaught and stops the program.

What happens if the condition is false?

If the condition is false, the throw statement is skipped and the function returns normally, but in this example, the condition is true (step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 2?

A 'Allowed'

B Error thrown: Too young

C No output

D Program continues normally

💡 Hint

See execution_table row 2, the error is thrown when age < 18.

At which step does the program stop due to an uncaught error?

A Step 1

B Step 2

C Step 3

D It never stops

💡 Hint

Check execution_table row 3, program stops because error is not caught.

If we call checkAge(20), what would change in the execution table?

A Condition false, function returns 'Allowed'

B Program stops at step 3

C Error thrown at step 2

D Error thrown at step 1

💡 Hint

Refer to variable_tracker and condition check in execution_table step 2.

Concept Snapshot

Throw errors with throw new Error('message')
Use if conditions to decide when to throw
If error not caught, program stops
Use try-catch to handle errors
Throwing interrupts normal flow
Errors carry messages for debugging

Full Transcript

This example shows how JavaScript throws errors using the throw statement. The function checkAge checks if the age is less than 18. If yes, it throws an error with the message 'Too young'. The execution table traces the steps: calling the function with 15, checking the condition, throwing the error, and stopping the program because there is no catch block. Variables like age are tracked through the steps. Key moments explain why the program stops and what happens if the condition is false. The quiz asks about the output at throwing, when the program stops, and what changes if age is 20. The snapshot summarizes how throwing errors works and how it affects program flow.