Unary operators in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to increase the value of count by 1 using a unary operator.

Javascript

let count = 5;
count[1];
console.log(count);

Drag options to blanks, or click blank then click option'

A-

B++

C+=

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' which decreases the value.

Using '+=' which is not a unary operator.

Using '-' which subtracts but is not unary increment.

The unary increment operator ++ increases the value by 1.

2fill in blank

medium

Complete the code to convert the string numStr to a number using a unary operator.

Javascript

let numStr = "42";
let num = [1]numStr;
console.log(typeof num);

Drag options to blanks, or click blank then click option'

A+

BNumber

CparseInt

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using parseInt which is a function, not a unary operator.

Using Number which is a function, not a unary operator.

Using '-' which negates the number but does not convert string to number.

The unary plus operator converts a string to a number.

3fill in blank

hard

Fix the error in the code by choosing the correct unary operator to negate the value of value.

Javascript

let value = 10;
let negated = [1]value;
console.log(negated);

Drag options to blanks, or click blank then click option'

A-

B+

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '++' which increments the value.

Using '+' which does not change the sign.

Using '--' which decrements the value.

The unary minus operator negates the value, turning 10 into -10.

4fill in blank

hard

Fill both blanks to create an object flags where keys are numbers and values are their negations using unary operators.

Javascript

const numbers = [1, 2, 3];
const flags = {};
for (const num of numbers) {
  flags[[1]] = [2];
}
console.log(flags);

Drag options to blanks, or click blank then click option'

Anum

B-num

Cnum.toString()

Dnum * -1

Attempts:

3 left

💡 Hint

Common Mistakes

Using number directly as key without converting to string.

Using multiplication instead of unary minus for negation.

Using 'num' as value which is not negated.

Keys must be strings, so num.toString() is used. Values are negated with unary minus -num.

5fill in blank

hard

Fill both blanks to create a new array results where each element is the negation of the numeric conversion of strings in arr.

Javascript

const arr = ["5", "-3", "10"];
const results = arr.map(item => [1][2]item);
console.log(results);

Drag options to blanks, or click blank then click option'

A-

B+

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using logical NOT ! which converts to boolean.

Not converting string to number before negation.

Placing operators incorrectly causing syntax errors.

First, unary plus + converts string to number, then unary minus - negates it. The empty string '' is used to separate operators correctly.

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