Iterating over arrays in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the simplest way to loop through all elements in an array in JavaScript?

Using a for loop with an index to access each element one by one.

beginner

How does the forEach method work on arrays?

It runs a function once for each element in the array, giving you access to the element and its index.

intermediate

What is the difference between for...of and for...in when iterating arrays?

for...of loops over the values (elements) of the array, while for...in loops over the keys (indexes). For arrays, for...of is usually preferred.

intermediate

Can you modify the original array elements inside a forEach loop?

Yes, if you access the array by index inside the forEach callback, you can change elements. But changing the parameter directly won't affect the array.

intermediate

What happens if you use break inside a forEach loop?

break does not work inside forEach. To stop early, use a regular for loop or for...of loop.

Which loop is best to use if you want to stop iterating an array early?

A for loop

B forEach method

C map method

D filter method

Only a regular for loop allows using break to stop early.

What does the for...of loop iterate over in an array?

A Properties

B Indexes

C Keys and values

D Values

for...of loops over the values (elements) of the array.

Which method runs a function on each array element but does NOT return a new array?

A forEach

B filter

C map

D reduce

forEach runs a function on each element but returns undefined.

What will this code output? <br> const arr = [1,2,3]; arr.forEach((x, i) => arr[i] = x * 2); console.log(arr);

A [1, 2, 3]

B [2, 4, 6]

C [undefined, undefined, undefined]

D Error

The array elements are doubled by modifying them inside forEach.

Which loop is NOT recommended for arrays because it iterates over all enumerable properties?

A for loop

B for...of

C for...in

D forEach

for...in loops over all enumerable properties, which can include inherited ones, so it's not ideal for arrays.

Explain three different ways to iterate over an array in JavaScript and when you might use each.

Think about control flow and readability.

Describe why you cannot use break inside a forEach loop and what you can do instead to stop iteration early.

Consider how forEach is a function call.