JS-Dev-101 sample questions

Question 1

A developer is building a financial dashboard that processes transaction lists. They need to calculate the total balance from an array of transaction objects, where each object contains a `type` ('credit' or 'debit') and an `amount`.

Which array method and implementation best achieves this single-value result?

Accepted Answer

transactions.reduce((acc, curr) => curr.type === 'credit' ? acc + curr.amount : acc - curr.amount, 0)

Answer

transactions.map((acc, curr) => curr.type === 'credit' ? acc + curr.amount : acc - curr.amount, 0)

Answer

transactions.filter((acc, curr) => curr.type === 'credit' ? acc + curr.amount : acc - curr.amount)

Answer

transactions.forEach((curr) => { let total = 0; return curr.type === 'credit' ? total + curr.amount : total - curr.amount })

Question 2

A developer encounters a bug where a variable declared inside a `for` loop is accessible outside the loop, causing unexpected behavior in subsequent logic. The code uses the `var` keyword.

Which explanation describes why this is happening?

Accepted Answer

Variables declared with var have function scope and are not limited to the block in which they are defined.

Answer

Variables declared with var have block scope and are hoisted to the top of the block.

Answer

The variable was implicitly declared as a global variable because strict mode was not enabled.

Answer

Loop variables are always global in JavaScript regardless of the keyword used.

Question 3

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

Answer

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

Answer

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

Question 4

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

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

Answer

Both operations succeed because objects are passed by reference.

Answer

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

Question 5

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

Answer

products.sort()

Answer

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

Question 6

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 transformer option

Answer

The map function

Question 7

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

Accepted Answer

null == undefined

Accepted Answer

!![]

Answer

NaN === NaN

Answer

false == 'false'

Question 8

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

Destructuring assignment

Answer

Rest parameters

Answer

Type coercion

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Free JS-Dev-101 sample questions

A developer encounters a bug where a variable declared inside a `for` loop is accessible outside the loop, causing unexpected behavior in subsequent logic. The code uses the `var` keyword. Which explanation describes why this is happening?

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?

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