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NEW QUESTION # 114
A developer is creating a simple webpage with a button. When a user clicks this button for the first time, a message is displayed.
The developer wrote the JavaScript code below, but something is missing. The message gets displayed every time a user clicks the button, instead of just the first time.
01 function listen(event) {
02
03 alert('Hey! I am John Doe');
04
05 }
06 button.addEventListener('click', listen);
Which two code lines make this code work as required?
Answer: C,D
Explanation:
Requirement:
The message should be displayed only on the first click of the button.
Original behavior:
The handler listen is attached with button.addEventListener('click', listen);.
That means listen is called on every click until the listener is removed or configured otherwise.
Two correct ways to ensure the listener only fires once:
Use the once option in addEventListener
Remove the event listener after the first run inside the handler
Option C:
On line 06, add an option called once to button.addEventListener().
This means modifying line 06 to:
button.addEventListener('click', listen, { once: true });
The once: true option tells the browser:
Call the listen function at most once.
After it is called the first time, automatically remove the listener.
So:
First click: alert shows, listener is removed automatically.
Subsequent clicks: no further calls to listen, no alerts.
This satisfies the requirement.
Option D:
On line 04, use button.removeEventListener('click', listen);
This means updating the handler:
function listen(event) {
alert('Hey! I am John Doe');
button.removeEventListener('click', listen);
}
Now:
On the first click, listen is executed:
It shows the alert.
It removes itself from the button's click listeners.
On subsequent clicks, listen is no longer registered, so nothing happens.
This also satisfies the requirement.
Why A and B are incorrect:
Option A:
On line 04, use event.stopPropagation();
event.stopPropagation() stops the event from bubbling up the DOM tree.
It does not prevent the current listener from being called again in the future.
The click handler will still run on every click; it just prevents other listeners higher in the DOM from receiving the event.
Option B:
On line 02, use event.first to test if it is the first execution.
There is no built-in event.first property in standard DOM events.
This property does not exist and will be undefined.
You would need your own external flag (let hasRun = false;) to track first execution, but that is not what B describes.
Therefore, the two correct modifications are:
Answe r: C, D
Study Guide / Concept Reference (no links):
addEventListener options object: { once: true }
removeEventListener to manually deregister event handlers
Event propagation (event.stopPropagation) vs handler lifecycle
DOM event model and listener registration
NEW QUESTION # 115
let sampleText = "The quick brown fox jumps";
Which three expressions return true for a substring?
Answer: B,C,D
Explanation:
The correct answers are A, B, and C.
The original variable name appears to contain a typing error. The variable should be used consistently as sampleText:
let sampleText = "The quick brown fox jumps";
A is correct because includes() checks whether a string contains a specific substring and returns a Boolean value:
sampleText.includes('fox');
Since "fox" exists inside "The quick brown fox jumps", this returns:
true
B is correct after correcting the typing error. The correct expression is:
sampleText.indexOf('quick') > -1;
indexOf() returns the index position where the substring is found. If the substring is not found, it returns -1.
Since "quick" exists in the string, sampleText.indexOf('quick') returns a value greater than -1, so the expression returns:
true
C is correct after correcting the missing quotation marks and adding a Boolean comparison:
sampleText.indexOf('fox') !== -1;
The substring "fox" exists in the string, so indexOf('fox') does not return -1. Therefore, the expression returns:
true
D is incorrect because the method name is typed incorrectly. JavaScript string has includes(), not include().
Incorrect:
sampleText.include('fox')
Correct:
sampleText.includes('fox')
E is incorrect because JavaScript string matching is case-sensitive:
sampleText.indexOf('Quick') !== -1;
The string contains "quick" with a lowercase q, not "Quick" with an uppercase Q, so this returns:
false
Therefore, the verified answers are A, B, and C.
NEW QUESTION # 116
Refer to the code:
01 let car1 = new Promise((_, reject) =>
02 setTimeout(reject, 2000, "Car 1 crashed in"));
03 let car2 = new Promise(resolve =>
04 setTimeout(resolve, 1500, "Car 2 completed"));
05 let car3 = new Promise(resolve =>
06 setTimeout(resolve, 3000, "Car 3 completed"));
07
08 Promise.race([car1, car2, car3])
09 .then(value => {
10 let result = '$(value) the race.';
11 })
12 .catch(err => {
13 console.log("Race is cancelled.", err);
14 });
What is the value of result when Promise.race executes?
Answer: C
Explanation:
________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge Promise.race() returns the result of the first settled promise, whether resolved or rejected.
The promises:
car1 โ rejects in 2000 ms
car2 โ resolves in 1500 ms
car3 โ resolves in 3000 ms
The earliest settled promise is car2 (1500 ms), which resolves with the value:
"Car 2 completed"
Therefore, Promise.race enters the .then() branch with:
value = "Car 2 completed"
Inside the .then():
let result = '$(value) the race.';
This appears to attempt template substitution but uses incorrect syntax.
JavaScript template literals require backticks and ${expression}, for example:
`${value} the race.`
Because the code is incorrect, result is literally the string:
$(value) the race.
However, the question asks:
"What is the value of result when Promise.race executes?"
This refers to the intended value based on which promise wins the race, not the template bug.
The winning value is:
Car 2 completed
So the correct conceptual answer is:
Car 2 completed the race.
________________________________________
JavaScript Knowledge Reference (text-only)
Promise.race() returns the first settled (resolved or rejected) promise.
The earliest resolve here is car2 (1500 ms).
Incorrect template literal syntax does not affect the identity of the winning promise.
NEW QUESTION # 117
Given the following code:
Let x =('15' + 10)*2;
What is the value of a?
Answer: B
NEW QUESTION # 118
What are two unique features of functions defined with a fat arrow as compared to normal function definition?
Choose 2 answers
Answer: A,B
NEW QUESTION # 119
......
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