Why built-in methods are useful in Javascript - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array map with built-in method

What is the output of this code?

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(Math.sqrt);
console.log(doubled);

Javascript

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(Math.sqrt);
console.log(doubled);

A [1, 1.4142135623730951, 1.7320508075688772, 2]

B [2, 4, 6, 8]

C [1, 4, 9, 16]

D TypeError

💡 Hint

Math.sqrt returns the square root of a number.

The map method applies Math.sqrt to each number, so the output is the square root of each element.

❓ Predict Output

intermediate

Output of String built-in method

What is the output of this code?

const text = 'Hello World';
const result = text.toUpperCase();
console.log(result);

Javascript

const text = 'Hello World';
const result = text.toUpperCase();
console.log(result);

A 'HELLO WORLD'

B 'hello world'

C 'Hello World'

D ReferenceError

💡 Hint

toUpperCase converts all letters to uppercase.

The toUpperCase method converts all letters in the string to uppercase.

❓ Predict Output

advanced

Output of Array filter with built-in method

What is the output of this code?

const values = [10, 15, 20, 25, 30];
const filtered = values.filter(x => x % 20 === 0);
console.log(filtered);

Javascript

const values = [10, 15, 20, 25, 30];
const filtered = values.filter(x => x % 20 === 0);
console.log(filtered);

A [10, 20, 30]

B [20]

C [20, 40]

D SyntaxError

💡 Hint

filter keeps elements where the function returns true.

Only 20 is divisible by 20 with no remainder, so the filtered array contains only 20.

❓ Predict Output

advanced

Output of Object.keys built-in method

What is the output of this code?

const obj = {a: 1, b: 2, c: 3};
const keys = Object.keys(obj);
console.log(keys);

Javascript

const obj = {a: 1, b: 2, c: 3};
const keys = Object.keys(obj);
console.log(keys);

A ['a:1', 'b:2', 'c:3']

B [1, 2, 3]

C ['a', 'b', 'c']

D TypeError

💡 Hint

Object.keys returns an array of property names.

Object.keys returns an array of the object's own property names as strings.

❓ Predict Output

expert

Output of chaining built-in methods

What is the output of this code?

const sentence = '  JavaScript is fun  ';
const result = sentence.trim().split(' ').filter(word => word.length > 2).join('-');
console.log(result);

Javascript

const sentence = '  JavaScript is fun  ';
const result = sentence.trim().split(' ').filter(word => word.length > 2).join('-');
console.log(result);

A 'JavaScript-is-fun'

B 'JavaScript--is--fun'

C 'JavaScript-is-'

D 'JavaScript-fun'

💡 Hint

trim removes spaces, split divides words, filter removes short words, join connects with dashes.

trim() removes leading/trailing spaces: 'JavaScript is fun'. split(' ') gives ['JavaScript', 'is', 'fun']. filter(word => word.length > 2) removes 'is' (length 2), leaving ['JavaScript', 'fun']. join('-') gives 'JavaScript-fun'.