Classes

Access Modifiers

Access modifiers show which parts of a class are meant for callers and which are internal.

private `private` marks a class member that should only be used inside the class.

Access Modifiers

depositCents
access.ts
Replay: real traced execution (multi-file project)
class Wallet {
    private cents: number;

    constructor(startingCents: number) {
        this.cents = startingCents;
    }

    deposit(cents: number): void {
        this.cents = this.cents + cents;
    }

    balance(): number {
        return this.cents;
    }
}

const depositCents: number = 250;
const wallet: Wallet = new Wallet(1000);
wallet.deposit(depositCents);

console.log(`balance=${wallet.balance()}`);

class Wallet {
    private cents: number;

    constructor(startingCents: number) {
        this.cents = startingCents;
    }

    deposit(cents: number): void {
        this.cents = this.cents + cents;
    }

    balance(): number {
        return this.cents;
    }
}

const depositCents: number = 100;
const wallet: Wallet = new Wallet(1000);
wallet.deposit(depositCents);

console.log(`balance=${wallet.balance()}`);

class Wallet {
    private cents: number;

    constructor(startingCents: number) {
        this.cents = startingCents;
    }

    deposit(cents: number): void {
        this.cents = this.cents + cents;
    }

    balance(): number {
        return this.cents;
    }
}

const depositCents: number = 500;
const wallet: Wallet = new Wallet(1000);
wallet.deposit(depositCents);

console.log(`balance=${wallet.balance()}`);

  1. depositCents ← 250, this.cents ← 1000, wallet ← [object Object]

    4    constructor(startingCents: number) {5        this.cents = startingCents1000;6    }78    deposit(cents: number): void {9        this.cents = this.cents + cents;10    }1112    balance(): number {13        return this.cents;14    }15}1617const depositCents→ 250: number = 250;  //@depositCents=100, 50018const wallet→ [object Object]: Wallet = new Wallet(1000);19wallet.deposit(depositCents250);
    values this step1000this.cents

  2. this.cents ← 1250

    5    this.cents = startingCents;6}78deposit(cents250: number): void {9    this.cents = this.cents→ 1250 + cents250;10}

  3. const wallet: Wallet = new Wallet(1000);

    17const depositCents: number = 250;  //@depositCents=100, 50018const wallet: Wallet = new Wallet(1000);19wallet.deposit(depositCents250);2021console.log(`balance=${wallet[object Object].balance()}`);

  4. balance(): number

    9    this.cents = this.cents + cents;10}1112balance(): number {13    return this.cents1250;14}

  5. console.log(`balance=${wallet.balance()}`);

    18const wallet: Wallet = new Wallet(1000);19wallet.deposit(depositCents);2021console.log(`balance=${wallet[object Object].balance()}`);
    outputbalance=1250

  1. depositCents ← 100, this.cents ← 1000, wallet ← [object Object]

    4    constructor(startingCents: number) {5        this.cents = startingCents1000;6    }78    deposit(cents: number): void {9        this.cents = this.cents + cents;10    }1112    balance(): number {13        return this.cents;14    }15}1617const depositCents→ 100: number = 100;18const wallet→ [object Object]: Wallet = new Wallet(1000);19wallet.deposit(depositCents100);
    values this step1000this.cents

  2. this.cents ← 1100

    5    this.cents = startingCents;6}78deposit(cents100: number): void {9    this.cents = this.cents→ 1100 + cents100;10}

  3. const wallet: Wallet = new Wallet(1000);

    17const depositCents: number = 100;18const wallet: Wallet = new Wallet(1000);19wallet.deposit(depositCents100);2021console.log(`balance=${wallet[object Object].balance()}`);

  4. balance(): number

    9    this.cents = this.cents + cents;10}1112balance(): number {13    return this.cents1100;14}

  5. console.log(`balance=${wallet.balance()}`);

    18const wallet: Wallet = new Wallet(1000);19wallet.deposit(depositCents);2021console.log(`balance=${wallet[object Object].balance()}`);
    outputbalance=1100

  1. depositCents ← 500, this.cents ← 1000, wallet ← [object Object]

    4    constructor(startingCents: number) {5        this.cents = startingCents1000;6    }78    deposit(cents: number): void {9        this.cents = this.cents + cents;10    }1112    balance(): number {13        return this.cents;14    }15}1617const depositCents→ 500: number = 500;18const wallet→ [object Object]: Wallet = new Wallet(1000);19wallet.deposit(depositCents500);
    values this step1000this.cents

  2. this.cents ← 1500

    5    this.cents = startingCents;6}78deposit(cents500: number): void {9    this.cents = this.cents→ 1500 + cents500;10}

  3. const wallet: Wallet = new Wallet(1000);

    17const depositCents: number = 500;18const wallet: Wallet = new Wallet(1000);19wallet.deposit(depositCents500);2021console.log(`balance=${wallet[object Object].balance()}`);

  4. balance(): number

    9    this.cents = this.cents + cents;10}1112balance(): number {13    return this.cents1500;14}

  5. console.log(`balance=${wallet.balance()}`);

    18const wallet: Wallet = new Wallet(1000);19wallet.deposit(depositCents);2021console.log(`balance=${wallet[object Object].balance()}`);
    outputbalance=1500

Follow the Deposit

  1. The wallet starts with 1000 cents.
  2. depositCents starts at 250.
  3. deposit adds 250 to the stored cents.
  4. balance() returns the new cents value.
  5. The program prints balance=1250. | depositCents | starting cents | balance | | --- | --- | --- | | 100 | 1000 | 1100 | | 250 | 1000 | 1250 | | 500 | 1000 | 1500 |

Exercise: access.ts

Reproduce balance=1250, then use depositCents 100 and 500 to predict each balance.