First and Last Occurrence of Element in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'First Occurrence of Element' mean in an array?

It means the position (index) where the element appears for the very first time when we look from the start of the array.

beginner

What does 'Last Occurrence of Element' mean in an array?

It means the position (index) where the element appears for the very last time when we look from the end of the array.

intermediate

Why is it useful to find the first and last occurrence of an element?

It helps us know the range where the element appears in the array, which is useful for counting how many times it appears or for searching efficiently.

beginner

What is a simple way to find the first occurrence of an element in JavaScript?

Use the array method indexOf(element), which returns the first index where the element is found or -1 if not found.

beginner

How can you find the last occurrence of an element in JavaScript?

Use the array method lastIndexOf(element), which returns the last index where the element is found or -1 if not found.

Which JavaScript method returns the first index of an element in an array?

A includes()

B lastIndexOf()

C find()

D indexOf()

The method indexOf() returns the first index where the element is found.

What does lastIndexOf() return if the element is not found in the array?

A 0

B -1

C null

D undefined

lastIndexOf() returns -1 when the element is not found.

If an element appears multiple times, what does indexOf() return?

A The first position of the element

B The last position of the element

C The count of the element

D All positions of the element

indexOf() returns the first position where the element appears.

Which method would you use to find the last occurrence of an element in an array?

A indexOf()

B find()

C lastIndexOf()

D filter()

lastIndexOf() returns the last position of the element in the array.

What is the output of ['a', 'b', 'a', 'c'].indexOf('a')?

A 0

B 1

C 2

D -1

The first 'a' is at index 0.

Explain how to find the first and last occurrence of an element in a JavaScript array.

Think about built-in array methods that give positions.

Why might knowing the first and last occurrence of an element be helpful in programming?

Consider how knowing positions helps with array operations.