Accessing array elements in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an array in JavaScript?

An array is a list that holds multiple values in order. You can think of it like a row of boxes, each with a number to find what’s inside.

beginner

How do you access the first element of an array named fruits?

Use fruits[0]. Arrays start counting at 0, so the first box is number 0.

beginner

What happens if you try to access an index that is outside the array length?

You get undefined because there is no value stored at that position.

intermediate

How can you find the last element of an array arr without knowing its length?

Use arr[arr.length - 1]. arr.length gives the number of items, so subtract 1 to get the last index.

intermediate

What is the difference between arr[1] and arr['1'] in JavaScript?

There is no difference. JavaScript converts the string '1' to the number 1 when accessing array elements.

What index do arrays in JavaScript start counting from?

A 1

B Depends on the array

C -1

D 0

Arrays always start at index 0, so the first element is at position 0.

What will arr[5] return if arr has 3 elements?

A undefined

B The 6th element

C Error

D null

Accessing an index outside the array length returns undefined, not an error.

How do you get the last element of an array myArray?

A myArray[myArray.length - 1]

B myArray[-1]

C myArray[myArray.length]

D myArray.last()

The last element is at index length minus 1 because counting starts at 0.

What does arr['2'] access in JavaScript?

A The element at index '2' as a string key

B An error because index must be a number

C The element at index 2

D The whole array

JavaScript converts string '2' to number 2 when accessing array elements.

If colors = ['red', 'green', 'blue'], what is colors[1]?

A 'red'

B 'green'

C 'blue'

D undefined

Index 1 is the second element, which is 'green'.

Explain how to access elements in a JavaScript array and what happens if you use an index outside the array.

Think about how you find items in a list by their position.

Describe how to get the last element of an array without knowing its length beforehand.

Remember the array length counts items, but indexes start at zero.

Practice

1. What is the index of the first element in a JavaScript array?

easy

A. -1

B. 1

C. 0

D. Depends on the array length

Solution

1. Step 1: Understand array indexing

   JavaScript arrays start counting elements from 0, not 1.

2. Step 2: Identify the first element's index

   The first element is always at index 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   First element index = 0 [OK]

Hint: Remember arrays start at zero, not one [OK]

Common Mistakes:

• Thinking first element index is 1
• Confusing negative indexes
• Assuming index depends on array length

2. Which of the following is the correct syntax to access the third element of an array named colors?

easy

A. colors{2}

B. colors[2]

C. colors[3]

D. colors(3)

Solution

1. Step 1: Recall array access syntax

   JavaScript arrays use square brackets with zero-based index to access elements.

2. Step 2: Calculate index for third element

   The third element is at index 2 (0,1,2).

3. Final Answer:

   colors[2] -> Option B

4. Quick Check:

   Third element index = 2 [OK]

Hint: Use square brackets and zero-based index [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using curly braces
• Using index 3 for third element

3. What will be the output of the following code?

const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits[1]);

medium

A. undefined

B. apple

C. cherry

D. banana

Solution

1. Step 1: Identify the array elements and indexes

   The array fruits has 'apple' at index 0, 'banana' at index 1, and 'cherry' at index 2.

2. Step 2: Access element at index 1

   fruits[1] returns 'banana'.

3. Final Answer:

   banana -> Option D

4. Quick Check:

   fruits[1] = banana [OK]

Hint: Remember index 1 is second element [OK]

Common Mistakes:

• Choosing first element instead
• Confusing index 1 with 2
• Expecting undefined for valid index

4. Find the error in this code snippet:

const numbers = [10, 20, 30];
console.log(numbers(1));

medium

A. Using parentheses instead of square brackets

B. Array is not declared properly

C. Index 1 is out of range

D. Missing semicolon

Solution

1. Step 1: Check array access syntax

   Arrays must be accessed with square brackets, not parentheses.

2. Step 2: Identify the error in the code

   numbers(1) is invalid syntax; correct is numbers[1].

3. Final Answer:

   Using parentheses instead of square brackets -> Option A

4. Quick Check:

   Array access needs [] not () [OK]

Hint: Use [] to access array elements, not () [OK]

Common Mistakes:

• Using () instead of []
• Thinking index 1 is invalid
• Ignoring syntax errors

5. Given the array const data = [5, 10, 15];, what will console.log(data[3]); output?

hard

A. undefined

B. 15

C. Error

D. 0

Solution

1. Step 1: Understand array length and valid indexes

   The array has elements at indexes 0, 1, and 2 only.

2. Step 2: Access index 3 which does not exist

   Accessing an index outside the array returns undefined in JavaScript.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   Out-of-range index returns undefined [OK]

Hint: Out-of-range indexes return undefined [OK]

Common Mistakes:

• Expecting error instead of undefined
• Confusing last element with index 3
• Assuming 0 is default value