← ACARA Digital Technologies Years 7–8

const for values that never change

Declares a variable whose value is set once and cannot be reassigned. Use it for fixed values like a maximum score. Changing it later causes a TypeError.

Part of: ACARA Digital Technologies Years 7–8 — Programming with Control Structures and Functions (JavaScript) — Module 11

Understanding const for values that never change in ACARA Digital Technologies Years 7–8

What it does: A fixed number of rounds is set once, so const is the right choice. The score goes up during the game, so it must be declared with let so it can be reassigned. The keyword const comes first, then the name, then = and the value. A const cannot be reassigned. JavaScript stops the program with a TypeError. If a value will change later, use let. A const would cause a TypeError when you update it. A const is set once, at the moment it is declared. You cannot leave it empty and fill it in later. A running total changes each time, so let is the correct keyword and const causes a TypeError.

Common mistakes: MAX_SCORE was declared with const, so it cannot be reassigned. This causes a TypeError. The line should cap the score: score = MAX_SCORE;. radius changes on the next line, so it must be declared with let. With const, radius = radius * 2 would cause a TypeError. It should be let radius = 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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Each concept is reinforced from every angle — learned, then applied, then tested.

Learning

Duo

Which line correctly declares a value that should never change?

MAX_SCORE const = 100;const MAX_SCORE = 100;

Matches

const MAX_SCORE = 100; MAX_SCORE = 200;

↓

MAX_SCORE is a const, so it cannot be reassigned. This causes a TypeError: Assignment to constant variable.

Applying

Meanings

Term

const

Meaning

Declares a variable whose value is set once and cannot be reassigned. Use it for fixed values like a maximum score. Changing it later causes a TypeError.

Practice

const maxScore = 100;

Testing

Gaps

MAXSCORE = 100;

Check Answer

Spots

3score += 15;

4MAX_SCORE = score;

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Quick Questions

1. Which line correctly declares a value that should never change?

A. const MAX_SCORE = 100; ✓

B. MAX_SCORE const = 100;

The keyword const comes first, then the name, then = and the value.

2. What happens when this code runs? const maxLives = 3; maxLives = 2;

A. maxLives becomes 2 with no error

B. TypeError: Assignment to constant variable ✓

A const cannot be reassigned. JavaScript stops the program with a TypeError.

3. Which variable is best declared with const?

A. The player's current score

B. The number of days in a week ✓

The days in a week never change, so use const. A score changes during a game, so it needs let.

4. Which variable is best declared with let?

A. let lives = 3; (it goes down when the player loses a life) ✓

B. const lives = 3; (it goes down when the player loses a life)

If a value will change later, use let. A const would cause a TypeError when you update it.

5. True or false: a const must be given its value when it is declared.

A. True: const PI = 3.14; ✓

B. False: const PI; can be given a value later

A const is set once, at the moment it is declared. You cannot leave it empty and fill it in later.

Match the Pairs

const MAX_SCORE = 100; → Creates a fixed value called MAX_SCORE that is set once and cannot be changed later

let lives = 3; → Creates a variable that can be given a new value later, such as when a life is lost

Use const for a value that should never change, like the number of rounds in a game → const ROUNDS = 5;

Use let for a value that will change as the program runs, like a player's score → let score = 0;

const MAX_SCORE = 100; MAX_SCORE = 200; → MAX_SCORE is a const, so it cannot be reassigned. This causes a TypeError: Assignment to constant variable.

const total = 0; total = total + 10; → total needs to change, but const does not allow reassignment. It should be declared with let instead.

Key Term

const

Declares a variable whose value is set once and cannot be reassigned. Use it for fixed values like a maximum score. Changing it later causes a TypeError.

Fill in the Blanks

1. const MAX_SCORE = 100;

Hint: Type the keyword that declares a value that can never be reassigned.

2. let lives = 3; // this value will change

Hint: Type the keyword for a variable whose value can be changed later.

3. const TAX = 10; console.log(5 * TAX);

Hint: Type the name of the constant that holds the fixed value.

4. const LIVES = 3; LIVES = 2; // TypeError

Hint: Type the name of the error JavaScript gives when you reassign a const.

5. const SCHOOL_YEARS = 6; // never changes

Hint: Type the keyword you choose for a fixed value that never changes.

Spot the Error

Question 1

A const MAX_SCORE = 100;

B let score = 90;

C score += 15;

D MAX_SCORE = score;

Line D — MAX_SCORE was declared with const, so it cannot be reassigned. This causes a TypeError. The line should cap the score: score = MAX_SCORE;

Question 2

A const PI = 3.14;

B const radius = 5;

C radius = radius * 2;

D console.log(PI * radius);

Line B — radius changes on the next line, so it must be declared with let. With const, radius = radius * 2 would cause a TypeError. It should be let radius = 5;

Question 3

A const TAX = 0.1;

B let price = 50;

C const total;

D console.log(total);

Line C — A const must be given its value when it is declared, because it can never be assigned later. It should be const total = price + price * TAX;

Question 4

A const MAX_LIVES = 3;

B let lives = MAX_LIVES;

C MAX_LIVES--;

D console.log(lives);

Line C — MAX_LIVES is a const, so it cannot be changed with --. This causes a TypeError. The changing variable lives should be reduced: lives--;

Question 5

A cosnt MAX_PLAYERS = 4;

B let joined = 2;

C console.log(MAX_PLAYERS - joined);

Line A — The keyword const is misspelled as cosnt. JavaScript does not recognise it, so it is a syntax error. It should be const MAX_PLAYERS = 4;

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