Iterating over objects in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of iterating object keys with for...in

What is the output of the following code?

Javascript

const obj = {a: 1, b: 2, c: 3};
let result = '';
for (const key in obj) {
  result += key;
}
console.log(result);

A "abc"

B "123"

C "a1b2c3"

D "undefined"

💡 Hint

for...in loops over keys of an object.

The for...in loop iterates over the keys of the object, so concatenating keys 'a', 'b', and 'c' results in 'abc'.

❓ Predict Output

intermediate

Output of Object.entries iteration

What is the output of this code snippet?

Javascript

const obj = {x: 10, y: 20};
let sum = 0;
for (const [key, value] of Object.entries(obj)) {
  sum += value;
}
console.log(sum);

A 30

B xy

C undefined

D 1020

💡 Hint

Object.entries returns key-value pairs as arrays.

Object.entries returns [['x',10], ['y',20]]. The loop sums values 10 + 20 = 30.

❓ Predict Output

advanced

Output of iterating object with Object.keys and map

What will this code output?

Javascript

const obj = {a: 1, b: 2, c: 3};
const result = Object.keys(obj).map(key => obj[key] * 2);
console.log(result);

A ["a2", "b4", "c6"]

B TypeError

C [1, 2, 3]

D [2, 4, 6]

💡 Hint

Object.keys returns keys, map transforms values.

Object.keys returns ['a','b','c']. Mapping each key to obj[key]*2 gives [2,4,6].

❓ Predict Output

advanced

Output of for...of with Object.values

What is the output of this code?

Javascript

const obj = {p: 5, q: 10};
let output = '';
for (const val of Object.values(obj)) {
  output += val + '-';
}
console.log(output);

A "510"

B "pq"

C "5-10-"

D SyntaxError

💡 Hint

Object.values returns values only.

Object.values returns [5,10]. The loop concatenates '5-' and '10-' resulting in '5-10-'.

❓ Predict Output

expert

Output of nested object iteration with destructuring

What is the output of this code?

Javascript

const data = {a: {x: 1}, b: {x: 2}};
let sum = 0;
for (const {x} of Object.values(data)) {
  sum += x;
}
console.log(sum);

A TypeError

B 3

C NaN

D undefined

💡 Hint

Object.values returns nested objects, destructuring extracts x.

Object.values(data) returns [{x:1}, {x:2}]. Destructuring {x} extracts values 1 and 2, summed to 3.