Assignment operators in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the = operator do in JavaScript?

The = operator assigns the value on the right to the variable on the left. For example, x = 5 sets the variable x to 5.

beginner

What is the difference between = and += operators?

= assigns a value directly, while += adds the right value to the left variable and then assigns the result back. For example, x += 3 means x = x + 3.

beginner

List three compound assignment operators in JavaScript besides +=.

Three common compound assignment operators are:
1. -= (subtract and assign)
2. *= (multiply and assign)
3. /= (divide and assign)

intermediate

What happens if you use x ||= y in JavaScript?

The ||= operator assigns y to x only if x is falsy (like null, undefined, 0, or ''). It's a shortcut for x || (x = y).

intermediate

Explain the difference between &= and ^= operators.

&= performs a bitwise AND between the variable and the right value, then assigns the result back.
^= performs a bitwise XOR (exclusive OR) between the variable and the right value, then assigns the result back.

What does the operator += do in JavaScript?

A Adds the right value to the left variable and assigns the result to the left variable

B Assigns the right value to the left variable without change

C Subtracts the right value from the left variable

D Multiplies the left variable by the right value

+= adds the right value to the left variable and stores the result back in the left variable.

Which operator assigns a value only if the left variable is falsy?

A +=

B ||=

C &&=

D *=

||= assigns the right value only if the left variable is falsy.

What does x &= y do?

A Performs bitwise OR between x and y, then assigns to x

B Adds x and y, then assigns to x

C Assigns y to x if x is falsy

D Performs bitwise AND between x and y, then assigns to x

&= performs bitwise AND and assigns the result to the left variable.

Which operator multiplies and assigns in one step?

A *=

B /=

C +=

D -=

*= multiplies the left variable by the right value and assigns the result back.

What is the result of let x = 5; x ||= 10;?

A x is 10

B x is 15

C x is 5

D Error

Since 5 is truthy, ||= does not assign 10 to x, so x remains 5.

Explain how compound assignment operators work in JavaScript with examples.

Think about how you can shorten 'x = x + 3' using a compound operator.

Describe the purpose and behavior of logical assignment operators like ||= and &&=.

Consider how these operators help assign values only under certain conditions.

Practice

1. What does the assignment operator += do in JavaScript?

easy

A. Adds the right value to the left variable and updates the variable

B. Subtracts the right value from the left variable

C. Assigns the right value to the left variable without change

D. Multiplies the left variable by the right value

Solution

1. Step 1: Understand the += operator

   The += operator adds the value on the right to the variable on the left.

2. Step 2: Effect on the variable

   It updates the left variable with the new sum, combining addition and assignment.

3. Final Answer:

   Adds the right value to the left variable and updates the variable -> Option A

4. Quick Check:

   x += y means x = x + y [OK]

Hint: Think: add then save back to the same variable [OK]

Common Mistakes:

• Confusing += with simple assignment =
• Thinking it subtracts or multiplies
• Believing it only adds without updating

2. Which of the following is the correct syntax to multiply a variable num by 5 and update it using an assignment operator?

easy

A. num =* 5;

B. num = *5;

C. num *= 5;

D. num = 5*;

Solution

1. Step 1: Identify the correct multiplication assignment syntax

   The correct operator to multiply and assign is *=, placed between variable and value.

2. Step 2: Check each option

   num *= 5; uses num *= 5; which is valid syntax. Others have incorrect operator placement or invalid syntax.

3. Final Answer:

   num *= 5; -> Option C

4. Quick Check:

   Use *= for multiply and assign [OK]

Hint: Remember operator goes between variable and value, no spaces inside [OK]

Common Mistakes:

• Writing =* instead of *=
• Adding spaces inside operator
• Using invalid symbols like 5*;

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

let x = 10;
x -= 3;
console.log(x);

medium

A. 13

B. 7

C. 3

D. 10

Solution

1. Step 1: Understand the initial value and operation

   Variable x starts at 10. The operator -= subtracts 3 from x.

2. Step 2: Calculate the new value

   10 - 3 = 7, so x becomes 7.

3. Final Answer:

   7 -> Option B

4. Quick Check:

   x -= 3 means x = x - 3 [OK]

Hint: Subtract then save result back to variable [OK]

Common Mistakes:

• Adding instead of subtracting
• Printing old value before update
• Confusing -= with =-

4. Find the error in this code snippet:

let count = 5;
count +== 2;
console.log(count);

medium

A. Console.log should be console.print

B. Variable count is not declared

C. Missing semicolon after declaration

D. The operator +== is invalid syntax

Solution

1. Step 1: Identify the operator used

   The code uses +== which is not a valid assignment operator in JavaScript.

2. Step 2: Correct operator usage

   The correct operator to add and assign is +=. So it should be count += 2;.

3. Final Answer:

   The operator +== is invalid syntax -> Option D

4. Quick Check:

   Use += not +== for addition assignment [OK]

Hint: Assignment operators have one = sign only [OK]

Common Mistakes:

• Using double equals with + (like +==)
• Confusing console.log with other functions
• Ignoring semicolon errors (usually optional)

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

let result = 2;
result *= 3;
result += 4;
result /= 2;

hard

A. 5

B. 7

C. 8

D. 10

Solution

1. Step 1: Multiply result by 3

   Starting with result = 2, after result *= 3, result becomes 6.

2. Step 2: Add 4 to result

   After result += 4, result becomes 6 + 4 = 10.

3. Step 3: Divide result by 2

   After result /= 2, result becomes 10 / 2 = 5.

4. Final Answer:

   5 -> Option A

5. Quick Check:

   2 * 3 + 4 then / 2 = 5 [OK]

Hint: Follow each operator step-by-step to update variable [OK]

Common Mistakes:

• Doing operations in wrong order
• Forgetting to update variable after each step
• Mixing up division and multiplication