JavaScript-Developer-I Exam Questions Tutorials

Event-driven Node-style APIs expose events via .on(eventName, handler).

To listen for error events:

server.on( ' error ' , (error) = > {

console.log( ' ERROR ' , error);

});

A and B assume methods .error or .catch that don’t exist on typical event emitters.

C only catches synchronous errors from server.start(), not async event-based errors.

The File API in browsers provides the FileReader object to read file contents selected from < input type= " file " > .

Important knowledge points:

new FileReader() creates a file-reading object.

.readAsDataURL(file) reads a file and produces a Base64 URL string.

The " load " event fires when the file has finished reading.

reader.result contains the data URL after reading completes.

Therefore, the correct implementation must:

Create a FileReader instance:

const reader = new FileReader();

Call:

reader.readAsDataURL(file);

Use the load event handler to assign the image preview:

preview.src = reader.result;

Option B is the only option that matches valid JavaScript File API usage.

Option A is incorrect because File is not a constructor for reading files.

Option C is incorrect because URL.createObjectURL(file) must be assigned directly as a URL, not used with reader.result.

JavaScript Knowledge References (text-only)

The file-reading interface in browsers is FileReader.

readAsDataURL() loads files as Base64 data URLs.

The load event indicates when the reader has finished and reader.result is available.

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Key details:

var has function scope , not block scope.

Declaration var msg is hoisted to the top of the function, but initialization only happens if the if condition is true.

Effectively, the function behaves like:

function getAvailableilityMessage(item) {

var msg; // hoisted declaration

if (getAvailableility(item)) {

msg = " Username available " ;

}

return msg;

}

Now, given:

getAvailableility( " newUserName " ) returns false.

Execution:

The if condition is false, so the body does not execute.

Therefore msg is declared but never assigned .

In JavaScript, an uninitialized variable that has been declared with var has the value undefined.

Thus:

return msg; // returns undefined

Why other options are wrong:

A: " newUserName " – msg never receives the parameter value; it ' s only set to " Username available " inside the if, which does not run.

B: " msg is not defined " – That kind of error occurs if msg were never declared. Here it is declared via var, so it is defined but undefined.

D: " Username available " – This would require the if branch to run, which it does not when getAvailableility(...) is false.

So the return value is:

undefined

Study Guide Concepts:

var hoisting and function scope

Uninitialized variables default to undefined

Control flow and conditional initialization

execute() returns a Promise that immediately calls reject(). So promise starts in a rejected state.

When a Promise is rejected and you chain .then() calls without rejection handlers, all those .then() callbacks are skipped until a .catch() is encountered:

promise

then(...) // skipped

then(...) // skipped

then(...) // skipped

catch(...) // executed

then(...); // executed after catch

Execution:

.then(() = > console.log( ' Resolved1 ' )) is skipped.

.then(() = > console.log( ' Resolved2 ' )) is skipped.

.then(() = > console.log( ' Resolved3 ' )) is skipped.

.catch(() = > console.log( ' Rejected ' )) runs and logs Rejected.

The .catch() returns a resolved Promise (no explicit return, so undefined), so the next .then() runs:

.then(() = > console.log( ' Resolved4 ' )) logs Resolved4.

Final output:

Rejected

Resolved4

This matches option D.