Unary operators in Javascript - Time & Space Complexity

Time Complexity: Unary operators

O(n)

Understanding Time Complexity

Unary operators perform simple actions on a single value. We want to see how their use affects the time it takes for code to run.

How does the number of unary operations grow as input size changes?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const numbers = [1, 2, 3, 4, 5];
for (let i = 0; i < numbers.length; i++) {
  numbers[i] = -numbers[i];
}
    

This code uses a unary minus operator to change each number to its negative inside a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Applying the unary minus operator to each element.
• How many times: Once for each element in the array.

How Execution Grows With Input

Each element gets processed once, so the work grows directly with the number of elements.

Input Size (n)	Approx. Operations
10	10 unary operations
100	100 unary operations
1000	1000 unary operations

Pattern observation: Doubling the input doubles the number of unary operations.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line with the number of items.

Common Mistake

[X] Wrong: "Unary operators run instantly and don't add to time complexity."

[OK] Correct: Even simple operations take time, and when repeated many times, they add up proportionally to input size.

Interview Connect

Understanding how simple operations inside loops affect performance helps you explain code efficiency clearly and confidently.

Self-Check

"What if we replaced the unary minus with a function call inside the loop? How would the time complexity change?"

Practice

1. Which of the following is a unary operator in JavaScript?

easy

A. ++

B. +=

C. &&

D. ===

Solution

1. Step 1: Understand unary operator definition

   A unary operator works with only one operand (value).

2. Step 2: Identify unary operators in options

   ++ increments a single variable by 1, so it is unary. += and && require two operands, === compares two values.

3. Final Answer:

   ++ -> Option A

4. Quick Check:

   Unary operator = ++ [OK]

Hint: Unary operators work on one value only, like ++ or -- [OK]

Common Mistakes:

• Confusing += as unary instead of binary
• Thinking && is unary when it's binary
• Mixing comparison operators with unary

2. Which of the following is the correct syntax to decrement variable x by 1 using a unary operator?

easy

A. --x-

B. x--

C. x-=

D. x--x

Solution

1. Step 1: Recall decrement unary operator syntax

   The decrement operator is written as -- before or after the variable.

2. Step 2: Check each option for correct syntax

   x-- is valid and decreases x by 1. x-= is incomplete syntax. --x- and x--x are invalid.

3. Final Answer:

   x-- -> Option B

4. Quick Check:

   Decrement syntax = x-- [OK]

Hint: Decrement uses -- placed before or after variable [OK]

Common Mistakes:

• Using x-= instead of x--
• Adding extra characters like x--x
• Placing -- incorrectly like --x-

3. What is the output of the following code?

let a = 5;
console.log(-a);

medium

A. -5

B. 5

C. NaN

D. undefined

Solution

1. Step 1: Understand unary minus operator

   The unary minus operator (-) changes the sign of the number.

2. Step 2: Apply unary minus to variable a

   a is 5, so -a becomes -5.

3. Final Answer:

   -5 -> Option A

4. Quick Check:

   Unary minus of 5 = -5 [OK]

Hint: Unary minus flips sign: positive to negative [OK]

Common Mistakes:

• Confusing unary minus with subtraction
• Expecting output 5 instead of -5
• Thinking it returns NaN or undefined

4. Find the error in this code snippet:

let count = 10;
count = ++;

medium

A. Syntax error due to missing semicolon

B. Incorrect use of decrement operator

C. Missing variable after ++ operator

D. No error, code runs fine

Solution

1. Step 1: Analyze the use of ++ operator

   The ++ operator must be used with a variable, either before or after it.

2. Step 2: Identify the error in the code

   Here, ++ is used alone without a variable, causing a syntax error.

3. Final Answer:

   Missing variable after ++ operator -> Option C

4. Quick Check:

   ++ needs a variable to work [OK]

Hint: ++ must be attached to a variable, not alone [OK]

Common Mistakes:

• Thinking ++ alone increments something
• Ignoring syntax errors from incomplete expressions
• Confusing missing semicolon as main error

5. Given the code below, what will be the value of result?

let x = 3;
let y = ++x + x++ + --x;
let result = y;

hard

A. 10

B. 13

C. 11

D. 12

Solution

1. Step 1: Evaluate ++x (pre-increment)

   ++x increases x from 3 to 4, returns 4.

2. Step 2: Evaluate x++ (post-increment)

   x++ returns current x (4), then increments x to 5.

3. Step 3: Evaluate --x (pre-decrement)

   --x decreases x from 5 to 4, returns 4.

4. Step 4: Calculate y

   y = 4 + 4 + 4 = 12.

5. Step 5: Check final value of result

   result = y = 12.

6. Final Answer:

   12 -> Option D

7. Quick Check:

   Pre/post increments affect value and order [OK]

Hint: Pre-increment changes value before use; post-increment after [OK]

Common Mistakes:

• Ignoring order of increments and decrements
• Mixing pre- and post-increment results
• Calculating without updating variable values