Unary operators in Javascript

Introduction

Unary operators help you change or check a single value quickly. They make your code shorter and easier to read.

When you want to add or subtract 1 from a number, like counting steps.

When you want to convert a string to a number before doing math.

When you want to check the type of a value to understand what it is.

When you want to flip a true/false value to its opposite.

When you want to delete a property from an object.

Syntax

Javascript

++variable
--variable
+value
-value
!value
delete object.property

Unary operators work with only one value.

Some unary operators change the value, others just check or convert it.

Examples

Increment operator adds 1 to the variable.

Javascript

let count = 5;
count++;  // count becomes 6

Unary plus converts a string to a number.

Javascript

let str = "123";
let num = +str;  // num is 123 as a number

Logical NOT flips true to false and vice versa.

Javascript

let flag = true;
let notFlag = !flag;  // notFlag is false

Delete removes a property from an object.

Javascript

let obj = {a: 1};
delete obj.a;  // obj is now {}

Sample Program

This program shows different unary operators: increment, decrement, unary plus, logical NOT, and delete.

Javascript

let x = 10;
console.log(x);      // 10
x++;
console.log(x);      // 11
x--;
console.log(x);      // 10

let str = "42";
console.log(+str);    // 42 (number)

let isTrue = false;
console.log(!isTrue); // true

let person = {name: "Anna", age: 25};
delete person.age;
console.log(person);  // { name: 'Anna' }

Important Notes

Increment (++) and decrement (--) operators change the variable's value by 1.

Unary plus (+) tries to convert a value to a number.

Logical NOT (!) flips true to false and false to true.

Delete removes a property from an object but does not affect variables.

Summary

Unary operators work with one value to change or check it.

They help write shorter and clearer code for simple tasks.

Common unary operators include ++, --, +, -, !, and delete.

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