Methods that change an object's properties
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. Which method line changes the object's own balance?
2. After account.deposit(50) runs on an account with balance 100, what is account.balance?
3. If you call account.deposit(50) twice on a balance of 100, what is the balance?
4. Which method checks the amount before it changes the balance?
5. A withdraw method has this line: this.balance -= amount; What does calling it do?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1this.balance += amount; | AThe + operator only works out a total and throws it away. It never stores the result, so this.balance does not change. | |
| 2if (amount > 0) { this.balance += amount; } | Bacc.deposit(50); acc.deposit(50); // balance is now 100 if it started at 0 | |
| 3A method changes the object's state by updating a property with this | CAdds the amount to the object's own balance property, so the object's state changes | |
| 4Calling a changing method twice applies the change twice, so the state builds up | DChecks the amount is valid before changing the property, so bad values leave the state unchanged | |
| 5deposit(amount) { balance += amount; } | Edeposit(amount) { this.balance += amount; } | |
| 6deposit(amount) { this.balance + amount; } | Fbalance has no this. in front of it, so the method does not update the object's property. It looks for a plain variable and causes an error. |
C
Definitions
this.balance += amount
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
deposit(amount) {
this.balance __ amount;
}
2.
if (amount __ this.balance) {
this.balance -= amount;
}
3.
withdraw(amount) {
____.balance -= amount;
}
4.
account.deposit(50);
account.deposit(50);
console.log(account._______);
5.
__ (amount > 0) {
this.balance += amount;
}
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
deposit(amount {
B
this.balance += amount;
C
}
Question 2
A
class Account {
B
deposit(amount) {
C
balance += amount;
D
}
E
}
Question 3
A
const acc = new Account(100);
B
acc.deposit(50);
C
acc.deposit(50);
D
console.log(acc.balance());
Question 4
A
function withdraw(amt) {
B
if (this.balance >= amt) {
C
this.balance -= amt;
D
}
E
}
Question 5
A
deposit(amount) {
B
if (amount <= 0) return;
C
this.balance += amount;
D
this.deposits + 1;
E
}
Answer Key
Methods that change an object's properties
Section A: Quick Questions
Q1
A — this.balance += amount;
Q2
B — 150
Q3
A — 200
Q4
A — if (amount > 0) { this.balance += amount; }
Q5
B — It lowers the object's balance each time it is called
Section B: Match the Pairs
Q1
this.balance += amount; → Adds the amount to the object's own balance property, so the object's state changes
Q2
if (amount > 0) { this.balance += amount; } → Checks the amount is valid before changing the property, so bad values leave the state unchanged
Q3
A method changes the object's state by updating a property with this → deposit(amount) { this.balance += amount; }
Q4
Calling a changing method twice applies the change twice, so the state builds up → acc.deposit(50); acc.deposit(50); // balance is now 100 if it started at 0
Q5
deposit(amount) { balance += amount; } → balance has no this. in front of it, so the method does not update the object's property. It looks for a plain variable and causes an error.
Q6
deposit(amount) { this.balance + amount; } → The + operator only works out a total and throws it away. It never stores the result, so this.balance does not change.
Section C: Definitions
Q1
this.balance += amount — A method line that updates the object's own property, so its state changes each time the method is called. Calling deposit(50) twice adds 50 twice.Section D: Fill in the Blanks
Q1
deposit(amount) {
this.balance += amount;
}Q2
if (amount <= this.balance) {
this.balance -= amount;
}Q3
withdraw(amount) {
this.balance -= amount;
}Q4
account.deposit(50);
account.deposit(50);
console.log(account.balance);Q5
if (amount > 0) {
this.balance += amount;
}Section E: Spot the Error
Q1
A — The closing bracket is missing from the parameter list. It should be deposit(amount) {.
Q2
C — Without this., JavaScript looks for a variable called balance, not the object's property. It should be this.balance += amount;.
Q3
D — balance is a property, not a method, so it must not be called with (). It should be console.log(acc.balance); and prints 200.
Q4
A — Methods inside a class are written without the function keyword. It should be withdraw(amt) {.
Q5
D — this.deposits + 1 works out a value but never stores it, so the property does not change. It should be this.deposits += 1;.