JavaScript-Developer-I Exam Dumps - Salesforce Developer Questions and Answers

React and Vue are:

Mature front-end JavaScript frameworks/libraries.

Have large communities.

Are widely supported and used in production across industries.

Appropriate for building browser-based Single Page Applications.

Express and Koa are server-side frameworks , not front-end SPA frameworks.

The scenario describes building a browser SPA, so the correct seasoned options are React and Vue.

JavaScript Knowledge References (text-only)

React and Vue are client-side frameworks for SPA development.

Express and Koa are backend frameworks for Node.js.

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The browser console (Developer Tools → Console) allows:

Running code unrelated to the page Developers can type arbitrary JavaScript not tied to the page’s logic. This is allowed → A is correct.

Viewing and modifying JavaScript code execution Using the Sources tab and console, developers can:

Inspect variables

Modify code at runtime

Set breakpoints Therefore → D is correct.

Viewing and manipulating the DOM From the console, developers can:

Query DOM nodes using selectors

Modify element properties and attributes

Insert or remove elements Therefore → E is correct.

Why the other options are incorrect:

B (security cookies) Many cookies have flags such as HttpOnly and Secure. Cookies with the HttpOnly flag cannot be viewed or modified via JavaScript or console. Therefore this is not guaranteed → incorrect.

C (performance report) The console itself does not provide a performance report. The Performance or Lighthouse tabs provide this, not the console. Therefore incorrect.

JavaScript Knowledge References (text-only)

Developer console can execute arbitrary JavaScript.

Console allows DOM inspection and manipulation.

Security cookies (HttpOnly) cannot be accessed via JavaScript.

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JavaScript consoles (browsers and Node) provide timing utilities:

console.time(label) – starts a timer with the given label.

console.timeEnd(label) – stops the timer and logs the total elapsed time.

console.timeLog(label) – logs the current elapsed time without stopping the timer.

To measure the time taken by each function separately while still using a single overall timer, you can log intermediate times:

console.time( ' Performance ' );

maybeAHeavyFunction();

console.timeLog( ' Performance ' , ' after maybeAHeavyFunction ' );

thisCouldTakeTooLong();

console.timeLog( ' Performance ' , ' after thisCouldTakeTooLong ' );

orMaybeThisOne();

console.timeLog( ' Performance ' , ' after orMaybeThisOne ' );

console.timeEnd( ' Performance ' );

Each console.timeLog( ' Performance ' ) call prints the elapsed time since console.time( ' Performance ' ) was started, so you can infer how long each function took.

Note: console.endTime in the original snippet is incorrect; the real method is console.timeEnd.

Why other options are incorrect:

B. console.trace()

Prints a stack trace, not timing information.

C. console.timeStamp()

Used with browser performance tools; it does not directly show elapsed times like timeLog or timeEnd.

D. console.getTime()

Not a standard console method.

Thus, the function that can be used to obtain timing between calls is:

Answer: A

Study Guide Concepts:

console.time, console.timeLog, console.timeEnd

Measuring performance and intermediate timings

Standard console debugging API methods

Browsers often cache JavaScript, CSS, and HTML. This means:

The developer may be running with the latest version (often with cache disabled in DevTools).

The tester may still be loading an older, cached version of the page where the background-change logic does not exist.

To ensure they are actually testing the latest deployed code:

Disabling cache (A) in the tester’s browser (usually via DevTools → Network → “Disable cache”) forces fresh loading of all resources on each refresh.

Clearing cache (C) removes older resources from the browser so the next load fetches fresh files from the server.

Options B and D do not directly help verify the tester is using the latest code:

B checks the developer’s own browser, which is not the environment showing the problem.

D suggests reworking the code before confirming whether the tester is actually loading the correct version, which is premature.

So the two actions that help produce accurate testing results are A and C.

Key JavaScript Promise rule:

try...catch does not catch asynchronous errors that occur in Promise chains.

Therefore, the correct way to handle errors in a Promise is:

promise.then(...).catch(...);

Analysis of options:

A and B: Both wrap asynchronous code in try...catch, which does not catch Promise rejections. try...catch only catches errors thrown synchronously inside the block.

C: Correct. A .catch() on the Promise chain handles any error from the Promise returned by requestPromise().

D: .finally() executes regardless of success or failure, but it does not receive the error object , so it cannot handle the error.

Thus the only valid solution is option C .

JavaScript Knowledge References (text-only)

Promise rejections must be handled with .catch().

try...catch only handles synchronous code.

.finally() does not receive a rejection reason.

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Arrow functions have two defining characteristics:

Implicit return when written as a single expression:

const fn = () = > 5; // returns 5 automatically

This is unique to arrow functions → A is correct.

Lexical this binding : Arrow functions do not create their own this. Instead, they use the this value from the surrounding scope → B is correct.

Incorrect options:

C is false: There is no special argument called parentThis.

D is the opposite of arrow function behavior: Normal functions create their own this; arrow functions do not.

JavaScript Knowledge References (text-only)

Arrow functions have lexical this binding.

Arrow functions support implicit return for single expressions.

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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.

Currently:

sayHello expects a value name and prints it.

world is a function that returns ' World ' .

sayHello(world); passes the function object itself, not the result of calling it.

So name inside sayHello is a function, not the string " World " .

To keep the call sayHello(world) yet get " World " , sayHello must call the function parameter:

const sayHello = (name) = > {

console.log( ' Hello ' , name());

};

Now:

sayHello(world) passes the function.

Inside sayHello, name() calls world(), which returns " World " .

The console logs " Hello World " .

Why the others are wrong:

B: Changing line 7 to }(); would attempt to IIFE the function definition and break the declaration.

C: sayHello(world)() would try to call the return value of sayHello, which is undefined, causing an error.

D: Changing world to a function declaration does not change the fact that it is passed as a function reference; sayHello still prints the function object, not ' World ' .

Evaluate each option:

A. Arguments for .then() are optional.

This is correct.

then() has the signature:

promise.then(onFulfilled?, onRejected?)

Both arguments (onFulfilled and onRejected) are optional.

If a callback is not supplied, JavaScript provides a default pass-through handler.

B. .then() cannot be added after a catch.

Incorrect.

Promises support chaining in any order:

promise

catch(...)

then(...);

After a catch, the chain continues normally.

C. .then() manipulates and returns the original promise.

Incorrect.

then() always returns a new promise , not the original one.

This is fundamental to promise chaining behavior.

D. Returning values in .then() is not necessary.

Incorrect.

If you want the next .then() in the chain to receive a value, the current .then() must explicitly return it:

then(value = > {

return value * 2; // passes to next .then()

})

If nothing is returned, the next .then() receives undefined.

Why A is correct

It is the only statement that accurately describes built-in Promise behavior:

then() accepts optional arguments.

JavaScript Knowledge References (text-only)

.then(onFulfilled?, onRejected?) accepts optional handlers.

Promise chaining creates new promises for each .then().

catch() can be followed by additional .then() calls.

Returning inside .then() passes values to the next step in the chain.

Comprehensive and Detailed Explanation:

Errors thrown inside setTimeout callbacks occur asynchronously, in a different tick of the event loop. A try...catch around setTimeout itself (as in D) cannot catch errors thrown later in the scheduled callback.

To catch errors from countSheep() when it’s called asynchronously, the try...catch must wrap the call inside the timeout callback:

setTimeout(function() {

try {

countSheep();

} catch (e) {

handleError(e);

}

}, 1000);

B uses finally incorrectly and references e out of scope.

C is not valid JavaScript syntax.

D’s catch can only handle synchronous errors thrown before setTimeout returns, not those thrown inside the callback.