JS-Dev-101 sample questions

Question 1

A developer needs to create a unique list of user IDs from a raw array that contains duplicates. The final result must be an array.

Which approach provides the most concise and performant solution?

Accepted Answer

let unique = [...new Set(rawArray)];

Answer

let unique = Object.keys(Object.fromEntries(rawArray.map(id => [id, id])));

Answer

let unique = rawArray.map(item => item);

Answer

let unique = rawArray.filter((item, index) => rawArray.indexOf(item) === index);

Question 2

Consider the following code snippet involving object mutation:

```javascript
const config = {
env: 'dev',
retries: 3
};
config.retries = 5;
config = { env: 'prod', retries: 1 };
```

What happens when this code executes?

Accepted Answer

It throws an error on the reassignment `config = ...` but allows the property mutation.

Answer

Both operations succeed because objects are passed by reference.

Answer

Both operations fail because `const` makes the object immutable.

Answer

It throws an error on `config.retries = 5` because `config` is constant.

Question 3

A developer needs to sort an array of product objects by price (ascending). The price is a numeric property.

`const products = [{id: 1, price: 100}, {id: 2, price: 5}, {id: 3, price: 25}];`

Which sort implementation works correctly?

Accepted Answer

products.sort((a, b) => a.price - b.price)

Answer

products.sort()

Answer

products.sort((a, b) => b.price - a.price)

Answer

products.sort((a, b) => a.price > b.price)

Question 4

A developer is parsing a JSON string received from an API. The string contains a date field in ISO format. The developer wants to automatically convert this string into a JavaScript Date object during parsing.

Which feature of `JSON.parse` should be used?

Accepted Answer

The reviver function

Answer

The replacer function

Answer

The map function

Answer

The transformer option

Question 5

Which of the following expressions evaluates to `true` in JavaScript? (Select TWO)

Accepted Answer

null == undefined

Accepted Answer

!![]

Answer

false == 'false'

Answer

NaN === NaN

Question 6

A developer writes the following function to create a greeting:

```javascript
function greet(name, greeting = 'Hello') {
return `${greeting}, ${name}!`;
}
```

What feature is being used for the `greeting` argument?

Accepted Answer

Default parameters

Answer

Rest parameters

Answer

Type coercion

Answer

Destructuring assignment

Question 7

A financial application calculates tax using the following code:

```javascript
let price = 100.50;
let taxRate = 0.08;
let total = price * taxRate;
console.log(total);
```

The output is `8.040000000000001` instead of `8.04`. What is the primary cause of this behavior?

Accepted Answer

Floating-point precision issues

Answer

Incorrect operator usage

Answer

Implicit string coercion

Answer

Integer overflow

Question 8

You are implementing a cache mechanism where keys must be objects (e.g., DOM elements) and you do not want to prevent garbage collection of these keys if they are removed from the DOM.

Which data structure is most appropriate?

Accepted Answer

WeakMap

Answer

Set

Answer

Map

Answer

Object

Free JS-Dev-101 sample questions

A developer needs to create a unique list of user IDs from a raw array that contains duplicates. The final result must be an array. Which approach provides the most concise and performant solution?

Consider the following code snippet involving object mutation: ```javascript const config = { env: 'dev', retries: 3 }; config.retries = 5; config = { env: 'prod', retries: 1 }; ``` What happens when this code executes?

A developer needs to sort an array of product objects by price (ascending). The price is a numeric property. `const products = [{id: 1, price: 100}, {id: 2, price: 5}, {id: 3, price: 25}];` Which sort implementation works correctly?

A developer is parsing a JSON string received from an API. The string contains a date field in ISO format. The developer wants to automatically convert this string into a JavaScript Date object during parsing. Which feature of `JSON.parse` should be used?

Which of the following expressions evaluates to `true` in JavaScript? (Select TWO)

A developer writes the following function to create a greeting: ```javascript function greet(name, greeting = 'Hello') { return `${greeting}, ${name}!`; } ``` What feature is being used for the `greeting` argument?

A financial application calculates tax using the following code: ```javascript let price = 100.50; let taxRate = 0.08; let total = price * taxRate; console.log(total); ``` The output is `8.040000000000001` instead of `8.04`. What is the primary cause of this behavior?

You are implementing a cache mechanism where keys must be objects (e.g., DOM elements) and you do not want to prevent garbage collection of these keys if they are removed from the DOM. Which data structure is most appropriate?

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