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| Section | Weight | Objectives |
|---|---|---|
| Asynchronous Programming | 13% | - Event loop and asynchronous code execution - Callback patterns - Promises and async/await |
| Testing | 7% | - Unit testing with Jest - Test-driven development principles - Automated testing strategies |
| Variables, Types, and Collections | 23% | - Type coercion and type conversion - Array manipulation (map, filter, reduce, forEach, find) - JSON parsing and stringification - String manipulation and Date objects - Variable declaration (var, let, const, scoping, hoisting) |
| Browser and Events | 17% | - DOM manipulation - Event handling, propagation, bubbling, and capturing - Browser Developer Tools |
| Server Side JavaScript | 8% | - Salesforce API integration - Modules and file operations - Node.js applications |
| Objects, Functions, and Classes | 25% | - JavaScript modules (export, import) - Function declarations, expressions, and arrow functions - ES6 Classes, constructors, methods, and inheritance - Higher-order functions and closures - Object creation, properties, and prototypal inheritance |
| Debugging and Error Handling | 7% | - try-catch blocks - Logging strategies and debugging techniques |
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質問 # 133
A developer copied a JavaScript object:
01 function Person() {
02 this.firstName = "John";
03 this.lastName = "Doe";
04 this.name = () => `${this.firstName},${this.lastName}`;
05 }
06
07 const john = new Person();
08 const dan = Object.assign({}, john);
09 dan.firstName = 'Dan';
How does the developer access dan's firstName, lastName?
正解：A
解説：
Person instances have:
firstName and lastName as string properties.
A name method that returns a combined string: `${this.firstName},${this.lastName}`.
Object.assign({}, john) creates a shallow copy of john into a new object, dan.
After:
dan.firstName = 'Dan';
dan.name() returns "Dan,Doe".
Analysis of options:
A: dan.firstName() and dan.lastName() are function calls, but firstName/lastName are strings, not functions → TypeError.
B: Calls the defined method and uses both names correctly.
C: dan.name is a function reference; you'd still need to call it: dan.name().
D: dan.firstName + dan.lastName is "DanDoe", no separator. It accesses the properties but not in the method the developer defined.
The intended way, using the provided API, is dan.name().
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質問 # 134
Refer to the code below:
01 let total = 10;
02 const interval = setInterval(() => {
03 total++;
04 clearInterval(interval);
05 total++;
06 }, 0);
07 total++;
08 console.log(total);
Considering that JavaScript is single-threaded, what is the output of line 08 after the code executes?
正解：C
解説：
Comprehensive and Detailed Explanation From Exact Extract JavaScript knowledge:
Synchronous execution order
JavaScript executes code in a single thread, following a well-defined order:
All synchronous code runs first, line by line.
Asynchronous callbacks (like those scheduled with setInterval or setTimeout) are placed into the event queue and executed only after the current call stack is empty.
Let's follow the code step by step:
Line 01:
let total = 10;
total is initialized with the value 10.
Line 02:
const interval = setInterval(() => {
total++;
clearInterval(interval);
total++;
}, 0);
setInterval schedules the callback function to run repeatedly after a delay of at least 0 milliseconds, but it does not run immediately. The callback is added to the timer queue and will be invoked after the current synchronous script finishes and the event loop gets to process timer callbacks.
At this point, interval holds the interval ID, but the callback has not executed yet.
Line 07:
total++;
This is still synchronous, so it runs before any scheduled callbacks.
total was 10, now it becomes 11.
Line 08:
console.log(total);
At this moment, the interval callback has still not run (because the event loop has not yet processed the timer queue).
So total is 11, and console.log(total); outputs 11.
Therefore, the value printed at line 08 is 11, making option A correct.
What happens after the log (for understanding, not affecting the answer) After the main script finishes, the event loop processes the timer callback for setInterval:
Callback:
() => {
total++; // from 11 to 12
clearInterval(interval); // cancels further executions
total++; // from 12 to 13
}
So eventually total becomes 13, but this happens after console.log(total) has already executed. Since the question asks specifically for the output at line 08, the asynchronous updates do not change that line's output.
Why other options are incorrect
Option B (12): This would require the callback to run before the log, which does not happen because asynchronous callbacks are queued and executed after the current stack finishes.
Option C (10): Ignores the total++ on line 07.
Option D (13): This is the final value after the callback finishes, but it occurs after the console.log line executes, not at the time line 08 runs.
JavaScript knowledge references (descriptive, no links):
JavaScript is single-threaded and uses an event loop with a call stack and task queues.
setInterval schedules callbacks to run asynchronously after a minimum delay; the callback never runs before the current synchronous code finishes.
Synchronous statements like total++ on line 07 execute before any queued interval callback.
質問 # 135
Given the code below:
01 const delay = async delay => {
02 return new Promise((resolve, reject) => {
03 console.log(1);
04 setTimeout(resolve, delay);
05 });
06 };
07
08 const callDelay = async () => {
09 console.log(2);
10 const yup = await delay(1000);
11 console.log(3);
12 };
13
14 console.log(4);
15 callDelay();
16 console.log(5);
What is logged to the console?
正解：A
解説：
Execution order:
Top-level code runs synchronously:
Line 14: console.log(4); → logs 4.
Line 15: callDelay(); is called.
Inside callDelay:
Line 9: console.log(2); → logs 2.
Line 10: await delay(1000);:
Calls delay(1000).
Inside delay(1000):
Line 3: console.log(1); → logs 1.
Line 4: setTimeout(resolve, delay); schedules resolve in 1000 ms.
delay returns a pending Promise. await pauses callDelay here and returns control to the event loop.
Back to top-level:
Line 16: console.log(5); → logs 5.
So synchronous log sequence is: 4, 2, 1, 5.
After ~1000 ms:
The setTimeout in delay resolves the Promise.
The await in callDelay resumes.
Line 11: console.log(3); → logs 3.
Final log order: 4 2 1 5 3.
Both A and B show the same sequence; one must be chosen, so A is correct.
Concepts: async/await flow, Promise resolution timing, event loop, and ordering of synchronous vs timer callbacks.
質問 # 136
At Universal Containers, every team has its own way of copying JavaScript objects. The code snippet shows an implementation from one team:
01 function Person() {
02 this.firstName = "John";
03 this.lastName = "Doe";
04 this.name = () => {
05 console.log('Hello ${this.firstName} ${this.lastName}');
06 }
07 }
08
09 const john = new Person();
10 const dan = JSON.parse(JSON.stringify(john)); // (intended deep copy)
11 dan.firstName = 'Dan';
12 dan.name();
(Original line 10 is logically intended to be JSON.parse(JSON.stringify(john)) to perform a JSON clone.) What is the output of the code execution?
正解：D
解説：
JSON.stringify(john) converts the john object into a JSON string.
When you JSON.parse that string back, you get a plain object:
Only data that can be represented in JSON is preserved (numbers, strings, booleans, arrays, plain objects).
Functions are not preserved and are dropped.
So dan is a plain object with properties firstName and lastName, but no name method.
Therefore, dan.name is undefined, and dan.name() throws:
TypeError: dan.name is not a function
The literal string interpolation inside console.log('Hello ${...}') is also wrong (single quotes), but the code never reaches that line.
質問 # 137
A developer publishes a new version of a package with bug fixes but no breaking changes. The old version number was 2.1.1.
What should the new package version number be based on semantic versioning?
正解：D
解説：
Semantic versioning: MAJOR.MINOR.PATCH
MAJOR: incompatible API changes.
MINOR: add functionality in a backward compatible manner.
PATCH: backward compatible bug fixes.
Here:
Bug fixes only, no breaking changes → increment PATCH.
From 2.1.1 to 2.1.2.
So the correct new version is 2.1.2.
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質問 # 138
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