Truthiness narrowing in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable value is truthy.

Typescript

if ([1]) {
  console.log("Value is truthy");
}

Drag options to blanks, or click blank then click option'

Avalue == null

B!value

Cvalue === false

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using negation like !value instead of value

Checking equality with false instead of truthiness

In TypeScript, simply using the variable name in the if condition checks if it is truthy.

2fill in blank

medium

Complete the code to narrow the type by checking if input is truthy before accessing its property.

Typescript

function printLength(input: string | null) {
  if ([1]) {
    console.log(input.length);
  }
}

Drag options to blanks, or click blank then click option'

Ainput.length > 0

Binput

Cinput === null

Dinput === undefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking input === null which is true only when input is null

Checking input.length > 0 without ensuring input is not null

Checking input directly in the if condition narrows the type to string because null is falsy.

3fill in blank

hard

Fix the error by completing the condition to narrow data to a truthy value before accessing data.value.

Typescript

function process(data: { value: number } | null) {
  if ([1]) {
    console.log(data.value);
  }
}

Drag options to blanks, or click blank then click option'

Adata !== null

Bdata.value !== null

Cdata.value > 0

Ddata === undefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking data.value !== null causes error if data is null

Checking data === undefined misses the null case

Checking data !== null ensures data is not null, so accessing data.value is safe.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths only for words longer than 3 characters.

Typescript

const words = ["apple", "cat", "banana", "dog"];
const lengths = { [1]: [2] for (const word of words) if (word.length > 3) };

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Aword

Bword.length

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping keys and values

Using word.length as key which is a number

The dictionary keys are the words themselves, and the values are their lengths. The condition filters words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a filtered object with uppercase keys and values greater than 0.

Typescript

const data = { a: 1, b: 0, c: 3 };
const filtered = { [1]: [2] for (const [[3], v] of Object.entries(data)) if (v > 0) };

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Ak.toUpperCase()

Bv

Ck

Dv.toString()

Attempts:

3 left

💡 Hint

Common Mistakes

Using v as the key variable

Not converting keys to uppercase

Using v.toString() instead of v

The keys are converted to uppercase, values are kept as numbers, and the loop variable is k for keys.