Typeof operator in type context in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with the same type as another variable using typeof.

Typescript

let name = "Alice";
let userName: [1] = "Bob";

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

Atypeof name

Btypeof "name"

Ctype name

Dtype of name

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'type name' instead of 'typeof name'.

Putting quotes around the variable name inside typeof.

In TypeScript, typeof used in a type context extracts the type of a variable. So typeof name means the type of the variable name.

2fill in blank

medium

Complete the code to declare a function parameter type using typeof operator on a variable.

Typescript

const age = 30;
function printAge(value: [1]) {
  console.log(value);
}

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

Atypeof 30

BAgeType

Ctypeof age

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using the literal type 'number' instead of typeof operator.

Using typeof with a value like 30 instead of the variable.

Using typeof age in the parameter type means the parameter must be the same type as age, which is number.

3fill in blank

hard

Fix the error by completing the type alias using typeof operator on a variable.

Typescript

const isActive = true;
type ActiveStatus = [1];

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

Atypeof isActive

Bboolean

Ctypeof true

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using the literal type 'boolean' instead of typeof operator.

Using 'Boolean' which is the constructor, not the type.

The correct way to create a type alias from a variable's type is to use typeof isActive. This extracts the type boolean from the variable.

4fill in blank

hard

Fill both blanks to create a type alias for the type of a variable and use it in a function parameter.

Typescript

const score = 100;
type ScoreType = [1];
function printScore(value: [2]) {
  console.log(value);
}

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

Atypeof score

Bnumber

CScoreType

Dtypeof 100

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' directly instead of the type alias.

Using typeof 100 instead of typeof score.

First, ScoreType is defined as typeof score to get the type of score. Then the function parameter uses ScoreType as its type.

5fill in blank

hard

Fill all three blanks to declare a variable, create a type alias using typeof, and use it in a function parameter.

Typescript

const isEnabled = false;
type EnabledType = [1];
let flag: [2] = false;
function toggle(value: [3]) {
  console.log(value);
}

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

Atypeof isEnabled

BEnabledType

Cboolean

Dtypeof false

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'boolean' directly instead of the type alias.

Using typeof false instead of typeof isEnabled.

We first create a type alias EnabledType using typeof isEnabled. Then we declare flag with type EnabledType. Finally, the function parameter uses EnabledType as its type.