Unary operators in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a unary operator in JavaScript?

A unary operator is an operator that works with only one operand. It performs an operation on a single value or variable.

beginner

What does the unary plus (+) operator do?

The unary plus (+) operator converts its operand to a number if it is not already one.

beginner

How does the unary negation (-) operator work?

The unary negation (-) operator converts its operand to a number and then negates it, changing its sign.

beginner

What is the purpose of the typeof operator?

The typeof operator returns a string indicating the type of the operand, such as 'number', 'string', or 'object'.

intermediate

What does the delete operator do in JavaScript?

The delete operator removes a property from an object. It returns true if the property was successfully deleted or did not exist.

Which of the following is a unary operator in JavaScript?

A ||

B &&

C ==

D +

The plus (+) operator used alone is unary, working on one operand. The others are binary operators.

What is the result of +"5" in JavaScript?

A 5 (number)

B "5" (string)

C NaN

D undefined

The unary plus converts the string "5" to the number 5.

What does typeof null return?

A "null"

B "boolean"

C "object"

D "undefined"

typeof null returns "object" due to a historical JavaScript quirk.

What does the delete operator do?

A Deletes a property from an object

B Deletes a variable

C Deletes a function

D Deletes an array element by index

The delete operator removes a property from an object, not variables or functions.

What is the result of -true in JavaScript?

A NaN

B -1

C 1

D false

true converts to 1, then unary negation changes it to -1.

Explain what unary operators are and give three examples in JavaScript.

Think about operators that work with only one value.

Describe how the delete operator works and when you might use it.

It is not used to delete variables.

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