JS-Dev-101学習指導、JS-Dev-101認証資格

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Salesforce JS-Dev-101 認定試験の出題範囲：

トピック出題範囲
トピック 1
  • 変数、型、コレクション：変数の宣言と初期化、文字列、数値、日付、配列、JSONの操作、型変換と真偽判定の理解について解説します。
トピック 2
  • ブラウザとイベント：DOM操作、イベント処理と伝播、ブラウザ固有のAPI、およびブラウザ開発者ツールを使用してコードの動作を検査する方法について説明します。
トピック 3
  • デバッグとエラー処理：適切なエラー処理手法、およびコンソールとブレークポイントを使用したコードのデバッグ方法について説明します。
トピック 4
  • サーバーサイドJavaScript：特定のシナリオにおけるNode.jsの実装、CLIコマンド、コアモジュール、およびパッケージ管理ソリューションを網羅しています。
トピック 5
  • オブジェクト、関数、クラス：ビジネス要件を満たすための関数、オブジェクト、クラスの実装に加え、モジュール、デコレータ、変数スコープ、実行フローの使用方法について解説します。

>> JS-Dev-101学習指導 <<

JS-Dev-101認証資格、JS-Dev-101問題トレーリング

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Salesforce Certified JavaScript Developer - Multiple Choice 認定 JS-Dev-101 試験問題 (Q76-Q81):

質問 # 76
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?

正解：A、D

解説：
Comprehensive and Detailed Explanation From Exact Extract JavaScript knowledge:
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:
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
________________________________________


質問 # 77
Which two console logs output NaN?

正解：B、C

解説：
Comprehensive and Detailed Explanation From JavaScript Knowledge:
Recall:
NaN stands for "Not-a-Number".
Arithmetic involving non-numeric values that cannot be converted to numbers often results in NaN.
Check each:
A . 10 / 0
In JavaScript, dividing a positive number by 0 results in Infinity, not NaN.
So this logs Infinity.
B . parseInt('two')
parseInt tries to parse a numeric prefix from the string.
'two' does not start with numeric digits, so parseInt('two') returns NaN.
The console log prints NaN.
C . 10 / Number('5')
Number('5') → 5.
10 / 5 → 2.
Logs 2, not NaN.
D . 10 / 'five'
'five' is coerced to a number using Number('five'), which is NaN.
10 / NaN → NaN.
Logs NaN.
So the console logs that output NaN are from B and D.
Relevant concepts: NaN, Infinity, parseInt, Number() coercion, arithmetic with invalid numeric values.
________________________________________


質問 # 78
JavaScript:
01 function Tiger() {
02 this.type = 'Cat';
03 this.size = 'large';
04 }
05
06 let tony = new Tiger();
07 tony.roar = () => {
08 console.log('They're great!');
09 };
10
11 function Lion() {
12 this.type = 'Cat';
13 this.size = 'large';
14 }
15
16 let leo = new Lion();
17 // Insert code here
18 leo.roar();
Which two statements could be inserted at line 17 to enable line 18?

正解：B、C

解説：
________________________________________
Comprehensive and Detailed Explanation From Exact Extract JavaScript Knowledge There are two valid ways to ensure leo.roar() exists:
Directly assign a roar function to leo (Option A).
Assigning a property to an instance object creates a new method on that instance.
leo.roar = () => { console.log('They're pretty good!'); };
After this, calling leo.roar() is valid.
Copy tony's properties into leo using Object.assign (Option B).
Object.assign(target, source) copies enumerable own properties from the source object into the target object.
Since tony.roar exists, executing:
Object.assign(leo, tony);
copies roar into leo, making leo.roar() valid.
Why the other answers are incorrect:
Option C:
Object.assign(leo, Tiger) copies properties from the function object Tiger, not from Tiger.prototype and not from a Tiger instance. Tiger (the function) has no roar property, so nothing useful is copied.
Option D:
leo.prototype is undefined because leo is an instance, not a constructor function. Only constructor functions have a .prototype property. This line would cause an error.
________________________________________
JavaScript Knowledge Reference (text-only)
Instances have their own properties and do not contain a .prototype property.
Object.assign(target, source) copies own enumerable properties of the source object.
Assigning a function as a property of an object creates a callable method.


質問 # 79
Refer to the following code block:
class Animal{
constructor(name){
this.name = name;
}
makeSound(){
console.log(`${this.name} ismaking a sound.`)
}
}
class Dog extends Animal{
constructor(name){
super(name)
this.name = name;
}
makeSound(){
console.log(`${this.name} is barking.`)
}
}
let myDog = new Dog('Puppy');
myDog.makeSound();
What is the console output?

正解：

解説：
Puppy is barking


質問 # 80
01 function Animal(size, type) {
02 this.type = type || 'Animal';
03 this.canTalk = false;
04 }
05
06 Animal.prototype.speak = function() {
07 if (this.canTalk) {
08 console.log("It spoke!");
09 }
10 };
11
12 let Pet = function(size, type, name, owner) {
13 Animal.call(this, size, type);
14 this.size = size;
15 this.name = name;
16 this.owner = owner;
17 }
18
19 Pet.prototype = Object.create(Animal.prototype);
20 let pet1 = new Pet();
Given the code above, which three properties are set for pet1?

正解：A、B、E

解説：
When pet1 = new Pet(); is created:
Inside Pet constructor:
Animal.call(this, size, type);
this.size = size;
this.name = name;
this.owner = owner;
Animal.call(this, size, type):
Sets this.type = type || 'Animal' → 'Animal' (because type is undefined).
Sets this.canTalk = false.
Then this.size, this.name, this.owner are set (to undefined since no args passed), but they do exist as properties.
So as own properties, pet1 has: type, canTalk, size, name, owner.
speak is defined on Animal.prototype, so pet1.speak exists by inheritance, but is not an own data property created in the constructor.
From the listed options, three important properties directly set by constructor logic and clearly used in behavior are:
canTalk
name
type
Thus, C, D, E.


質問 # 81
......

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