Array Deletion at Beginning in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens when you delete the first element of an array in JavaScript?

The first element is removed, and all other elements shift one position to the left to fill the gap.

beginner

Which JavaScript method removes the first element from an array?

The shift() method removes the first element and returns it, changing the original array.

intermediate

What is the time complexity of deleting the first element in a JavaScript array?

It is O(n) because all elements after the first must be shifted one position to the left.

intermediate

How does deleting the first element of an array compare to deleting the last element in terms of performance?

Deleting the first element (shift()) is slower because it shifts all elements, while deleting the last element (pop()) is faster and O(1).

beginner

Show the JavaScript code to delete the first element from array <code>let arr = [10, 20, 30]</code> and print the updated array.

Code:<br><code>let arr = [10, 20, 30];<br>arr.shift();<br>console.log(arr); // Output: [20, 30]</code>

Which method removes the first element of an array in JavaScript?

A pop()

B shift()

C slice()

D splice()

The shift() method removes the first element. pop() removes the last element.

What is the time complexity of deleting the first element of an array?

A O(n)

B O(1)

C O(log n)

D O(n^2)

Deleting the first element requires shifting all other elements, so it takes O(n) time.

After running arr.shift() on [5, 10, 15], what is the new array?

A [5, 10, 15]

B [15]

C [10, 15]

D []

The first element 5 is removed, leaving [10, 15].

Which method is faster for removing an element from an array?

A slice()

B shift()

C Both are equally fast

D pop()

pop() removes the last element in O(1) time, faster than shift() which is O(n).

What does shift() return when called on an empty array?

A undefined

B null

C 0

D Throws error

shift() returns undefined if the array is empty.

Explain how deleting the first element of an array affects the array structure and performance.

Think about what happens to all elements after the first.

Write JavaScript code to remove the first element from an array and print the updated array.

Use the method that removes the first element and then console.log.

Practice

1. What does the shift() method do to an array in JavaScript?

easy

A. Adds an element at the end

B. Removes the last element and returns it

C. Adds an element at the beginning

D. Removes the first element and returns it

Solution

1. Step 1: Understand the shift() method

   The shift() method removes the first element from an array and returns that element.

2. Step 2: Compare with other array methods

   pop() removes the last element, unshift() adds at the beginning, and push() adds at the end.

3. Final Answer:

   Removes the first element and returns it -> Option D

4. Quick Check:

   shift() removes first element [OK]

Hint: Remember: shift = remove from start [OK]

Common Mistakes:

• Confusing shift() with pop()
• Thinking shift() adds elements
• Mixing shift() with unshift()

2. Which of the following is the correct syntax to remove the first element from an array arr in JavaScript?

easy

A. arr.shift()

B. arr.delete(0)

C. arr.unshift()

D. arr.pop()

Solution

1. Step 1: Identify the method to remove first element

   The method to remove the first element is shift().

2. Step 2: Check other options

   pop() removes last element, unshift() adds elements at start, and delete is not a valid array method.

3. Final Answer:

   arr.shift() -> Option A

4. Quick Check:

   Use shift() to remove first element [OK]

Hint: Use arr.shift() to remove first element [OK]

Common Mistakes:

• Using pop() instead of shift()
• Trying to use delete() on arrays
• Confusing unshift() with shift()

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

let arr = [10, 20, 30, 40];
let removed = arr.shift();
console.log(arr);
console.log(removed);

medium

A. [20, 30, 40] and 10

B. [10, 20, 30] and 40

C. [20, 30, 40] and 40

D. [10, 20, 30] and 10

Solution

1. Step 1: Apply shift() on the array

   shift() removes the first element (10) from arr and returns it.

2. Step 2: Check the updated array and removed value

   After removal, arr becomes [20, 30, 40], and removed stores 10.

3. Final Answer:

   [20, 30, 40] and 10 -> Option A

4. Quick Check:

   shift() removes first element 10 [OK]

Hint: shift() returns removed first element [OK]

Common Mistakes:

• Expecting last element removed
• Confusing returned value with array
• Not updating array after shift()

4. Find the error in the following code that tries to remove the first element from array nums:

let nums = [5, 10, 15];
nums.remove(0);
console.log(nums);

medium

A. remove() removes last element, not first

B. remove() is not a valid array method

C. remove() requires two arguments

D. Code is correct and works fine

Solution

1. Step 1: Check the method used

   The method remove() does not exist for JavaScript arrays.

2. Step 2: Identify correct method to remove first element

   To remove the first element, use shift() instead of remove().

3. Final Answer:

   remove() is not a valid array method -> Option B

4. Quick Check:

   Use shift() to remove first element, not remove() [OK]

Hint: Use shift(), not remove(), to delete first element [OK]

Common Mistakes:

• Using remove() instead of shift()
• Assuming remove() exists in JS arrays
• Not checking method names carefully

5. Given an array data = [1, 2, 3, 4, 5], you want to remove the first two elements efficiently. Which code snippet correctly achieves this?

hard

A. data.shift(2);

B. data.pop(); data.pop();

C. data.splice(0, 2);

D. data.slice(2);

Solution

1. Step 1: Understand each method's effect

   shift(2) is invalid; shift() removes one element from start and takes no arguments. splice(0, 2) removes two elements starting at index 0 in one call. pop() removes from end, not start. slice(2) returns a new array without first two elements but does not modify original.

2. Step 2: Choose the best method to remove first two elements

   splice(0, 2) removes first two elements efficiently and modifies the original array.

3. Final Answer:

   data.splice(0, 2); -> Option C

4. Quick Check:

   splice(0, 2) removes first two elements [OK]

Hint: Use splice(0, 2) to remove first two elements at once [OK]

Common Mistakes:

• Using pop() which removes from end
• Using slice() which does not modify original array
• Using shift(2) which is invalid