Objects – for...in Loop
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. Does for...in loop over the keys of an object?
2. Is 'of' used instead of 'in' for looping over an object's keys?
3. Must the loop variable be declared with let before 'in'?
4. Does obj[key] access the value for the current key in the loop?
5. Does obj.key access the value using the loop variable's value?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1for (let key in person) { ... } | Afor (let key in obj) { ... } | |
| 2console.log(person[key]); | BWrong - use obj[key], not obj.key, to access the value | |
| 3Loops over each property name (key) of an object | CAccesses the value for the current key using bracket notation | |
| 4Retrieves a value using the loop's key variable | DLoops over each key in the person object | |
| 5for (let key of obj) { ... } | EWrong - use 'in', not 'of', to loop over an object's keys | |
| 6console.log(obj.key); | Fobj[key] |
C
Definitions
for...in
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
___ (let key in person) {
2.
for (let key __ person) {
3.
for (___ color in car) {
4.
___ (let prop in obj) {
5.
for (let prop __ obj) {
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
For (let k in a) {
B
let a = {x: 1};
C
console.log(k);
D
}
Question 2
A
let b = {y: 2};
B
for (let k of b) {
C
console.log(k);
D
}
Question 3
A
let c = {z: 3};
B
for (let k in c) {
C
console.log(c.k);
D
}
Question 4
A
let d = {a: 1};
B
for (let k in d) {
C
console.log(k);
D
]
Question 5
A
let e = {b: 2};
B
for (let k in e) {
C
console.log(k);
D
console.log(e[k]);
E
]
Answer Key
Objects – for...in Loop
Section A: Quick Questions
Q1
A — Yes
Q2
B — No
Q3
B — Yes
Q4
A — Yes
Q5
A — No
Section B: Match the Pairs
Q1
for (let key in person) { ... } → Loops over each key in the person object
Q2
console.log(person[key]); → Accesses the value for the current key using bracket notation
Q3
Loops over each property name (key) of an object → for (let key in obj) { ... }
Q4
Retrieves a value using the loop's key variable → obj[key]
Q5
for (let key of obj) { ... } → Wrong - use 'in', not 'of', to loop over an object's keys
Q6
console.log(obj.key); → Wrong - use obj[key], not obj.key, to access the value
Section C: Definitions
Q1
for...in — Loops through each key (property name) of an object, one at a time.Section D: Fill in the Blanks
Q1
for (let key in person) {Q2
for (let key in person) {Q3
for (let color in car) {Q4
for (let prop in obj) {Q5
for (let prop in obj) {Section E: Spot the Error
Q1
A — 'for' must be lowercase - a capital F causes an error.
Q2
B — Use 'in', not 'of', to loop through an object's keys.
Q3
C — Use c[k], not c.k, to access the value using the loop variable.
Q4
D — The block must close with }, not ].
Q5
E — The block must close with }, not ].