Index

Objects and Arrays

1. Which method is used to add one or more elements to the end of an array?
  1. push()
  2. pop()
  3. shift()
  4. unshift()
Answer 1. `push()`
2. What will const arr = [1, 2, 3]; arr.pop(); console.log(arr); output?
  1. [1, 2]
  2. [1, 2, 3]
  3. [2, 3]
  4. [ ]
Answer 1. `[1, 2]`
3. How do you remove the first element from an array in JavaScript?
  1. arr.pop()
  2. arr.shift()
  3. arr.unshift()
  4. arr.splice(0, 1)
Answer 2. `arr.shift()`
4. What will const arr = [1, 2, 3]; arr.unshift(0); console.log(arr); output?
  1. [1, 2, 3, 0]
  2. [0, 1, 2, 3]
  3. [1, 2, 3]
  4. undefined
Answer 2. `[0, 1, 2, 3]`
5. Which method would you use to join all elements of an array into a string, separated by a specified separator?
  1. join()
  2. split()
  3. concat()
  4. slice()
Answer 1. `join()`
6. How can you create a new array with all elements that pass a test provided by a function?
  1. forEach()
  2. map()
  3. filter()
  4. reduce()
Answer 3. `filter()`
7. What is the output of const arr = [1, 2, 3]; const doubled = arr.map(x => x * 2); console.log(doubled);?
  1. [1, 2, 3]
  2. [2, 4, 6]
  3. [0, 1, 4]
  4. [4, 6, 8]
Answer 2. `[2, 4, 6]`
8. Which method is used to check if at least one element in an array meets a condition?
  1. every()
  2. find()
  3. some()
  4. includes()
Answer 3. `some()`
9. What will the following code output? const arr = [1, 2, 3, 4]; console.log(arr.slice(1, 3));
  1. [1, 2, 3]
  2. [2, 3, 4]
  3. [2, 3]
  4. [3, 4]
Answer 3. `[2, 3]`
10. Which method would you use to flatten a nested array in JavaScript?
  1. flat()
  2. reduce()
  3. concat()
  4. splice()
Answer 1. `flat()`
11. How do you create an object in JavaScript?
  1. let obj = { };
  2. let obj = ( );
  3. let obj = [ ];
  4. let obj = newObject();
Answer 1. `let obj = { };`
12. What will const obj = {a: 1, b: 2}; console.log(obj.a); output?
  1. undefined
  2. 1
  3. 2
  4. null
Answer 2. `1`
13. Which method can you use to convert an object’s properties to an array of key-value pairs?
  1. Object.values()
  2. Object.keys()
  3. Object.entries()
  4. Object.assign()
Answer 3. `Object.entries()`
14. What will const obj = {a: 1, b: 2}; delete obj.a; console.log(obj); output?
  1. {a: 1, b: 2}
  2. {a: null, b: 2}
  3. {b: 2}
  4. undefined
Answer 3. `{b: 2}`
15. What is the correct way to check if a property exists in an object?
  1. obj.hasOwnProperty('key')
  2. obj.contains('key')
  3. obj.exists('key')
  4. obj.has('key')
Answer 1. `obj.hasOwnProperty('key')`
16. How would you iterate over all keys in an object using for...in?
let obj = { a: 1, b: 2 };
for (/* what goes here? */) { console.log(key); }
  1. let i in obj
  2. const key in obj
  3. key in obj
  4. for obj[key]
Answer 2. `const key in obj`
17. Which method merges properties of two objects into one?
  1. Object.join()
  2. Object.merge()
  3. Object.assign()
  4. Object.push()
Answer 3. `Object.assign()`
18. How would you retrieve all keys of an object as an array?
  1. Object.keys()
  2. Object.values()
  3. Object.entries()
  4. Object.get()
Answer 1. `Object.keys()`
19. What will const obj = {a: 1, b: {c: 3}}; console.log(obj.b.c); output?
  1. undefined
  2. 3
  3. {c: 3}
  4. 1
Answer 2. `3`
20. Which method creates a shallow copy of an object?
  1. Object.clone()
  2. Object.assign()
  3. Object.spread()
  4. Object.create()
Answer 2. `Object.assign()`
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