JavaScript Arrays, Iterables, and Collections

Question 1

When should I favor array methods (like map, filter, reduce) over traditional for loops?

Answer 1

For concise, readable transformations, filtering, aggregations, and simple iterations. They are declarative and maintainable. For loops offer more control and may be faster for large arrays or complex operations.

Question 2

What are the advantages and disadvantages of using array methods instead of for loops?

Answer 2

Advantages: Concise, readable, declarative, chainable. Disadvantages: Potentially less performant on large datasets/complex logic, harder to debug complex chains.

Question 3

What’s the difference between a shallow copy and a deep copy of an array?

Answer 3

Shallow Copy: New array, but elements are references to same objects. Changes to objects affect both arrays. Deep Copy: Completely new array with copies of all objects. Changes to objects only affect the copied array.

Question 4

How can you create a deep copy of an array in JavaScript (and what are the limitations)?

Answer 4

JSON.parse(JSON.stringify(array)) - simple but doesn’t work with functions, undefined, dates, or circular refs. _.cloneDeep(array) (Lodash) - robust. Recursive function - most control but complex.

Question 5

What methods create a shallow copy?

Answer 5

Spread syntax (…), slice(), Object.assign() when applied to arrays (performs shallow copy)

Question 6

Why is immutability important in JavaScript?

Answer 6

Makes code more predictable, easier to debug, avoids unexpected side effects. Essential for state management in React and Redux.

Question 7

What’s the difference between mutating and non-mutating array methods?

Answer 7

Mutating: Modify original array directly (push, pop, shift, unshift, splice, sort, reverse). Non-Mutating: Create new array or value without changing original (slice, concat, map, filter, reduce).

Question 8

Should you prefer mutating or non-mutating methods?

Answer 8

Favor non-mutating methods whenever possible to promote immutability and avoid unexpected side effects.

Question 9

Give examples of mutating array methods.

Answer 9

push(), pop(), shift(), unshift(), splice(), sort(), reverse(), fill()

Question 10

Give examples of non-mutating array methods.

Answer 10

slice(), concat(), map(), filter(), reduce(), flat(), flatMap()

Question 11

When would you choose to use an Object over an Array?

Answer 11

When storing key-value pairs with known string/Symbol keys, and you want direct access via dot notation.

Question 12

When would you choose to use a Set over an Array?

Answer 12

When you need to store only unique values and quickly check for existence of a value.

Question 13

When would you choose to use a Map over an Object?

Answer 13

When keys can be any data type (including objects), insertion order must be preserved, and frequent add/delete/update operations are needed.

Question 14

What are the key differences between a Map and a WeakMap?

Answer 14

Map: keys any type, strong refs, iterable, has .size. WeakMap: keys must be objects, weak refs, not iterable, no .size. WeakMap keys allow garbage collection.

Question 15

What are the key differences between a Set and a WeakSet?

Answer 15

Set: values any type, strong refs, iterable, has .size. WeakSet: values must be objects, weak refs, not iterable, no .size. WeakSet values allow garbage collection.

Question 16

When should you use a WeakMap?

Answer 16

When associating data with objects (as keys) without preventing garbage collection. Useful for caching, DOM metadata, private data in classes.

Question 17

When should you use a WeakSet?

Answer 17

When tracking a collection of objects without preventing garbage collection. Useful for private class data, marking objects as ‘processed’, etc.

Question 18

Why don’t WeakMap and WeakSet have a .size property or support iteration?

Answer 18

Because garbage collector can reclaim objects at any time, making size unpredictable and iteration unreliable.