Objects – for...in Loop

Loops through each key (property name) of an object, one at a time.

Part of: JavaScript Coding without HTML and CSS — Module 34

Understanding Objects – for...in Loop in JavaScript

What it does: This is exactly what a for...in loop does. Bracket notation with the variable name gets each property's value. Use 'in', not 'of', when looping over an object's properties. Just like other loops, the variable needs let (or const/var). Dot notation only accesses a literal property named 'key' - use obj[key] instead. JavaScript is case-sensitive, so 'for' must be lowercase. 'of' is used for arrays and other iterables, not plain objects.

Common mistakes: 'for' must be lowercase - a capital F causes an error. Use 'in', not 'of', to loop through an object's keys.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 31–40) and word search (Modules 21–52).

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Learning

Duo

Does for...in loop over the keys of an object?

YesNo

Matches

for (let key of obj) { ... }

↓

Wrong - use 'in', not 'of', to loop over an object's keys

Applying

Meanings

Term

for...in

Meaning

Loops through each key (property name) of an object, one at a time.

Practice

let person = {name: "Sam", age: 12};

Testing

Gaps

(let key in person) {

Check Answer

Spots

1For (let k in a) {

2let a = {x: 1};

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Quick Questions

1. Does for...in loop over the keys of an object?

A. Yes ✓

B. No

for...in iterates over each property name (key) in the object.

2. Is 'of' used instead of 'in' for looping over an object's keys?

A. Yes

B. No ✓

Use 'in', not 'of', when looping over an object's properties.

3. Must the loop variable be declared with let before 'in'?

A. No

B. Yes ✓

Just like other loops, the variable needs let (or const/var).

4. Does obj[key] access the value for the current key in the loop?

A. Yes ✓

B. No

Bracket notation with the loop variable retrieves each property's value.

5. Does obj.key access the value using the loop variable's value?

A. No ✓

B. Yes

Dot notation only accesses a literal property named 'key' - use obj[key] instead.

Match the Pairs

for (let key in person) { ... } → Loops over each key in the person object

console.log(person[key]); → Accesses the value for the current key using bracket notation

Loops over each property name (key) of an object → for (let key in obj) { ... }

Retrieves a value using the loop's key variable → obj[key]

for (let key of obj) { ... } → Wrong - use 'in', not 'of', to loop over an object's keys

console.log(obj.key); → Wrong - use obj[key], not obj.key, to access the value

Key Term

for...in

Loops through each key (property name) of an object, one at a time.

Fill in the Blanks

1. for (let key in person) {

Hint: Type the keyword that starts a for loop.

2. for (let key in person) {

Hint: Type the keyword that connects the variable to the object's keys.

3. for (let color in car) {

Hint: Type the keyword used to declare the loop variable.

4. for (let prop in obj) {

Hint: Type the keyword that starts a for loop.

5. for (let prop in obj) {

Hint: Type the keyword that connects the variable to the object's keys.

Spot the Error

Question 1

A For (let k in a) {

B let a = {x: 1};

C console.log(k);

D }

Line A — 'for' must be lowercase - a capital F causes an error.

Question 2

A let b = {y: 2};

B for (let k of b) {

C console.log(k);

D }

Line B — Use 'in', not 'of', to loop through an object's keys.

Question 3

A let c = {z: 3};

B for (let k in c) {

C console.log(c.k);

D }

Line C — Use c[k], not c.k, to access the value using the loop variable.

Question 4

A let d = {a: 1};

B for (let k in d) {

C console.log(k);

D ]

Line D — The block must close with }, not ].

Question 5

A let e = {b: 2};

B for (let k in e) {

C console.log(k);

D console.log(e[k]);

E ]

Line E — The block must close with }, not ].

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