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| Section | Weight | Objectives |
|---|---|---|
| Topic 1: Asynchronous Programming | 13% | - Event loop and asynchronous code execution - Callback patterns - Promises and async/await |
| Topic 2: Variables, Types, and Collections | 23% | - Type coercion and type conversion - String manipulation and Date objects - JSON parsing and stringification - Variable declaration (var, let, const, scoping, hoisting) - Array manipulation (map, filter, reduce, forEach, find) |
| Topic 3: Server Side JavaScript | 8% | - Modules and file operations - Node.js applications - Salesforce API integration |
| Topic 4: Debugging and Error Handling | 7% | - try-catch blocks - Logging strategies and debugging techniques |
| Topic 5: Objects, Functions, and Classes | 25% | - Object creation, properties, and prototypal inheritance - ES6 Classes, constructors, methods, and inheritance - Higher-order functions and closures - JavaScript modules (export, import) - Function declarations, expressions, and arrow functions |
| Topic 6: Browser and Events | 17% | - Browser Developer Tools - Event handling, propagation, bubbling, and capturing - DOM manipulation |
| Topic 7: Testing | 7% | - Unit testing with Jest - Test-driven development principles - Automated testing strategies |
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NEW QUESTION # 131
A developer wants to create a simple image upload using the File API.
HTML:
<input type="file" onchange="previewFile()">
<img src="" height="200" alt="Image preview..." />
JavaScript:
01 function previewFile() {
02 const preview = document.querySelector('img');
03 const file = document.querySelector('input[type=file]').files[0];
04 // line 4 code
05 reader.addEventListener("load", () => {
06 preview.src = reader.result;
07 }, false);
08 // line 8 code
09 }
Which code in lines 04 and 08 allows the selected local image to be displayed?
Answer: C
Explanation:
________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge 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 Reference (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.
NEW QUESTION # 132
Refer to the code:
01 const exec = (item, delay) =>
02 new Promise(resolve => setTimeout(() => resolve(item), delay));
03
04 async function runParallel() {
05 const [result1, result2, result3] = await Promise.all(
06 [exec('x', '100'), exec('y', '500'), exec('z', '100')]
07 );
08 return `parallel is done: ${result1}${result2}${result3}`;
09 }
Which two statements correctly execute runParallel()?
Answer: A,C
Explanation:
________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge Facts about JavaScript Promises:
runParallel() is declared async, which means it always returns a Promise.
Promises are consumed using .then(), .catch(), and .finally().
There is no .done() method in native JavaScript Promises.
The async keyword cannot be placed before a function call (option A is invalid syntax).
Analysis of each option:
A . async runParallel().then(data);
Invalid syntax. async cannot prefix an expression.
B . Valid. Calls the function and attaches a .then() handler.
C . Invalid. .done() is not part of JavaScript Promise API.
D . Valid. Calls runParallel() and chains .then().
Therefore the correct answers are B and D.
________________________________________
JavaScript Knowledge Reference (text-only)
async functions return Promises.
Promises use .then() to retrieve resolved values.
.done() is not part of the standard Promise interface.
NEW QUESTION # 133
01 let a = "*";
02 let b = "**";
/ 7 x = 3;
04 console.log(a);
What is displayed when the code executes?
Answer: D
NEW QUESTION # 134
A developer wrote the following code:
01 let x = object.value;
02
03 try {
04 handleObjectValue(x);
05 } catch(error) {
06 handleError(error);
07 }
The developer has a getNextValue function to execute after handleObjectValue(), but does not want to execute getNextValue() if an error occurs. How can the developer change the code to ensure this behavior?
Answer: C
Explanation:
Requirement:
getNextValue() should run only if handleObjectValue(x) does not throw an error.
If an error occurs, handleError(error) should run, and getNextValue() should be skipped.
Option B:
try {
handleObjectValue(x);
getNextValue();
} catch (error) {
handleError(error);
}
Behavior:
If handleObjectValue(x) succeeds (no error):
Execution continues to getNextValue();.
If handleObjectValue(x) throws:
Control jumps directly to catch, getNextValue() is skipped.
handleError(error); is called.
This matches the requirement perfectly.
Why others are incorrect:
A: } then { is invalid JavaScript syntax.
C:
try {
handleObjectValue(x);
} catch (error) {
handleError(error);
}
getNextValue();
getNextValue() is called after the try...catch regardless of whether an error occurred. Not acceptable.
D: finally always runs:
try {
handleObjectValue(x);
} catch (error) {
handleError(error);
} finally {
getNextValue();
}
getNextValue() will execute in both success and error cases.
Therefore, the correct approach is B.
Concepts: try/catch control flow, finally semantics, placing code inside vs outside try/catch blocks.
NEW QUESTION # 135
A Node.js server library uses events and callbacks. The developer wants to log any issues the server has at boot time.
Which code logs an error with an event?
Answer: D
Explanation:
console.log('ERROR', error);
}
Explanation:
________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge Node.js event-based modules use the EventEmitter pattern.
The correct syntax for listening to events is:
emitter.on('eventName', callback)
The server library emits an 'error' event, which must be listened to using .on.
Option analysis:
A: .catch is for Promises, not EventEmitters.
B: .error is not an EventEmitter method.
C: Correct. Listens to the 'error' event.
D: try...catch only captures synchronous errors, not event-based asynchronous errors.
Therefore, the correct answer is option C.
________________________________________
JavaScript Knowledge Reference (text-only)
The EventEmitter API uses on(event, handler) to listen for events.
Errors emitted asynchronously cannot be caught with try...catch.
The 'error' event is standard for Node.js modules to signal operational errors.
NEW QUESTION # 136
......
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