The array length property
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. let pets = ['cat', 'dog', 'fish'];
Which gives the number of items in pets?
2. let scores = [10, 20, 30, 40];
Which expression gets the LAST item?
3. What does this print?
let list = [];
console.log(list.length);
4. let games = ['chess', 'tag'];
games.push('soccer');
What is games.length now?
5. An array has 5 items. What is the index of its last item?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1scores[scores.length - 1] | AThe last index is length - 1, so this reads past the end and gives undefined. Write colours[colours.length - 1]. | |
| 2let items = []; console.log(items.length); | Blet count = names.length; | |
| 3length is a property, so it is written without brackets to give the number of items | Clength is a property, not a method, so it must not have brackets. Write fruits.length. | |
| 4The length grows when an item is added to the array | DGets the last item in the scores array, because the last index is always length - 1 | |
| 5let total = fruits.length(); | Elet pets = ['cat']; pets.push('dog'); console.log(pets.length); | |
| 6let last = colours[colours.length]; | FPrints 0, because an empty array has a length of 0 |
C
Definitions
array.length
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
let scores = [5, 8, 9];
console.log(scores.______);
2.
let scores = [5, 8, 9];
console.log(scores[scores.length - _]);
3.
let list = [];
console.log(list.length === _);
4.
let pets = ["cat"];
pets.push("dog");
console.log(pets.length === _);
5.
let nums = [4, 7, 2];
for (let i = 0; i _ nums.length; i++)
console.log(nums[i]);
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
let items = {};
B
items.push('pen');
C
items.push('ruler');
D
console.log(items.length);
Question 2
A
function lastItem(list) {
B
return arr[arr.length - 1];
C
}
D
console.log(lastItem([4, 8]));
Question 3
A
const days = ['Mon', 'Tue'];
B
days.push('Wed');
C
console.log(days.length());
Question 4
A
const a = [5, 10, 15];
B
const last = a[a.length];
C
console.log(last);
Question 5
A
const queue = [];
B
queue.push('Sam');
C
console.log(queue.lenght);
Answer Key
The array length property
Section A: Quick Questions
Q1
A — pets.length
Q2
A — scores[scores.length - 1]
Q3
A — 0
Q4
B — 3
Q5
B — 4
Section B: Match the Pairs
Q1
scores[scores.length - 1] → Gets the last item in the scores array, because the last index is always length - 1
Q2
let items = [];
console.log(items.length); → Prints 0, because an empty array has a length of 0
Q3
length is a property, so it is written without brackets to give the number of items → let count = names.length;
Q4
The length grows when an item is added to the array → let pets = ['cat'];
pets.push('dog');
console.log(pets.length);
Q5
let total = fruits.length(); → length is a property, not a method, so it must not have brackets. Write fruits.length.
Q6
let last = colours[colours.length]; → The last index is length - 1, so this reads past the end and gives undefined. Write colours[colours.length - 1].
Section C: Definitions
Q1
array.length — A property (no brackets) giving how many items are in an array. The last index is length - 1, an empty array has length 0, and length grows as items are added.Section D: Fill in the Blanks
Q1
let scores = [5, 8, 9];
console.log(scores.length);Q2
let scores = [5, 8, 9];
console.log(scores[scores.length - 1]);Q3
let list = [];
console.log(list.length === 0);Q4
let pets = ["cat"];
pets.push("dog");
console.log(pets.length === 2);Q5
let nums = [4, 7, 2];
for (let i = 0; i < nums.length; i++)
console.log(nums[i]);Section E: Spot the Error
Q1
A — {} makes an empty object, not an array, so push() fails and items.length is undefined. An empty array is written [] and starts with length 0.
Q2
A — The parameter is named list, but the body uses arr, so arr is not defined. The parameter must be arr to match arr.length.
Q3
C — length is a property, not a method, so it has no brackets. It should be days.length, which gives 3.
Q4
B — The last index is length - 1 because indexes start at 0. a[a.length] reads past the end and gives undefined. It should be a[a.length - 1].
Q5
C — length is misspelled as lenght, so it gives undefined. It should be queue.length, which prints 1 after one push.