← ACARA Digital Technologies Years 7–8

Shorthand updates: +=, -= and ++

A shorthand update. Operators like +=, -=, ++ and -- change a variable using its own value, so total += price means total = total + price. count++ adds 1 and count-- takes away 1.

Part of: ACARA Digital Technologies Years 7–8 — Programming with Control Structures and Functions (JavaScript) — Module 19

Understanding Shorthand updates: +=, -= and ++ in ACARA Digital Technologies Years 7–8

What it does: The ++ operator is the quickest way to add 1 to a variable and it means the same as count = count + 1. += adds the value on its right to the variable. The operator is written +=, with the plus first. =+ is not a shorthand update. ++ is shorthand for count = count + 1. It always adds exactly 1. -- is the decrement operator and it takes away 1. A single - on its own is not a valid statement. score += 5 means score = score + 5, so 10 + 5 = 15. The old value is kept and added to.

Common mistakes: A const variable cannot be changed, so count++ would cause a TypeError. Use let count = 0; for a variable that gets updated. The operator is += (plus first, then equals). =+ just assigns positive 5, so total becomes 5 instead of 15. It should be total += 5;.

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This module's key terms are also in a printable crossword (Modules 562–580) and word search (Modules 522–580).

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Learning

Duo

Which line does the same job as total = total + price;?

total =+ price;total += price;

Matches

total =+ price;

↓

The symbols are the wrong way round. =+ just assigns a positive price to total instead of adding it. It should be +=.

Applying

Meanings

Term

total += price

Meaning

A shorthand update. Operators like +=, -=, ++ and -- change a variable using its own value, so total += price means total = total + price. count++ adds 1 and count-- takes away 1.

Practice

let total = 10;

Testing

Gaps

total 5;

Check Answer

Spots

1const count = 0;

2count++;

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Quick Questions

1. Which line does the same job as total = total + price;?

A. total += price; ✓

B. total =+ price;

+= adds the value on its right to the variable. The operator is written +=, with the plus first. =+ is not a shorthand update.

2. What does count++; do to the variable count?

A. Adds 2 to count

B. Adds 1 to count ✓

++ is shorthand for count = count + 1. It always adds exactly 1.

3. Which statement takes 1 away from lives?

A. lives--; ✓

B. lives-;

-- is the decrement operator and it takes away 1. A single - on its own is not a valid statement.

4. let score = 10; score += 5; What is score now?

A. 15 ✓

B. 5

score += 5 means score = score + 5, so 10 + 5 = 15. The old value is kept and added to.

5. Which is the same as x = x + 1;?

A. x+1;

B. x++; ✓

x++ changes x itself. x+1; works out an answer but never stores it back in x.

Match the Pairs

total += price; → Adds the value of price to total and stores the answer back in total

count--; → Takes 1 away from count

Add 1 to the variable count using the shortest form → count++;

Write score = score - 5 in its shorthand form → score -= 5;

total =+ price; → The symbols are the wrong way round. =+ just assigns a positive price to total instead of adding it. It should be +=.

let count = 0; count+; → count+ is not a valid operator. To add 1 you need two plus signs, count++, or you can write count += 1.

Key Term

total += price

A shorthand update. Operators like +=, -=, ++ and -- change a variable using its own value, so total += price means total = total + price. count++ adds 1 and count-- takes away 1.

Fill in the Blanks

1. let total = 0; total += 5; console.log(total);

Hint: Type the shorthand operator that adds a value to the variable itself.

2. let lives = 3; lives--; console.log(lives);

Hint: Type the operator that takes 1 away from the variable.

3. let count = 0; count = count + 1; count++;

Hint: Type the variable name that the longhand update must use on the right side, just like count++ does.

4. const price = 8; let money = 20; money -= price;

Hint: Type the shorthand operator that subtracts the price from the variable itself.

5. const price = 6; let total = 0; total += price;

Hint: Type the name of the variable that is being updated by the shorthand operator.

Spot the Error

Question 1

A const count = 0;

B count++;

C console.log(count);

Line A — A const variable cannot be changed, so count++ would cause a TypeError. Use let count = 0; for a variable that gets updated.

Question 2

A let total = 10;

B total =+ 5;

C console.log(total);

Line B — The operator is += (plus first, then equals). =+ just assigns positive 5, so total becomes 5 instead of 15. It should be total += 5;

Question 3

A let score = 0;

B score += 10;

C score+;

D console.log(score);

Line C — The increase operator needs two plus signs. A single + is a syntax error. It should be score++;

Question 4

A let total;

B total += 4;

C total += 6;

D console.log(total);

Line A — Without a starting value, total is undefined, and undefined + 4 gives NaN. A running total must start at 0: let total = 0;

Question 5

A let lives = 3;

B lives =- 1;

C console.log(lives);

Line B — The operator is -= (minus first, then equals). =- assigns -1, so lives becomes -1 instead of 2. It should be lives -= 1;

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