How assignment compatibility is checked in Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with type number.

Typescript

let age: [1] = 30;

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using string type when assigning a number.

Using boolean type for numeric values.

The variable age is declared with type number, so it can only hold numeric values.

2fill in blank

medium

Complete the code to assign a string value to a variable declared as string.

Typescript

let name: string = [1];

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

A25

B"Alice"

Ctrue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning a number or boolean to a string variable.

Forgetting to use quotes around text.

The value assigned to name must be a string, so it needs to be in quotes like "Alice".

3fill in blank

hard

Fix the error by completing the code to assign a compatible type.

Typescript

let isActive: boolean = [1];

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

Afalse

Bnull

C1

D"true"

Attempts:

3 left

💡 Hint

Common Mistakes

Using string "true" instead of boolean true.

Using numbers like 1 or 0 for boolean.

The variable isActive expects a boolean value. Only false is a valid boolean here.

4fill in blank

hard

Fill both blanks to create a compatible assignment between two interfaces.

Typescript

interface Person { name: string; age: number; }
interface Employee extends Person { [1]: string; }
let emp: Employee = { name: "Bob", age: 25, [2]: "Manager" };

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

Aposition

Bsalary

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Using different property names in interface and object.

Using a property not declared in the interface.

The Employee interface adds a position property. The object emp must include this property with a string value.

5fill in blank

hard

Fill all three blanks to complete a function that checks assignment compatibility with union types.

Typescript

function printId(id: [1]) {
  if (typeof id === [2]) {
    console.log("ID is a number: " + id);
  } else if (typeof id === [3]) {
    console.log("ID is a string: " + id);
  }
}

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

Anumber | string

B"number"

C"string"

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect type strings in typeof checks.

Not matching the union type in the function parameter.

The function accepts an id that can be a number or string. The typeof checks must match the string literals "number" and "string".

Practice

1. In TypeScript, what does assignment compatibility primarily check between two types?

easy

A. If the types have the same number of methods

B. If the variable names are the same

C. If the values are equal at runtime

D. If the properties and their types match between the source and target types

Solution

1. Step 1: Understand assignment compatibility concept

   TypeScript checks if the shape (properties and their types) of one type fits into another.

2. Step 2: Compare properties and types

   It does not check variable names or runtime values but the structure of types.

3. Final Answer:

   If the properties and their types match between the source and target types -> Option D

4. Quick Check:

   Shape matching = Properties and types [OK]

Hint: Think of matching shapes, not names or values [OK]

Common Mistakes:

• Confusing variable names with type compatibility
• Thinking runtime values affect assignment compatibility
• Assuming method count matters instead of properties

2. Which of the following is the correct syntax to assign an object of type {name: string} to a variable of type {name: string; age?: number} in TypeScript?

easy

A. let person: {name: string; age: number} = {age: 30};

B. let person: {name: string; age?: number} = {name: 'Alice'};

C. let person: {name: string; age: number} = {name: 'Alice'};

D. let person: {name: string} = {name: 'Alice', age: 30};

Solution

1. Step 1: Check optional property usage

   The target type has an optional property age?, so the source object can omit it.

2. Step 2: Verify assignment compatibility

   Assigning {name: 'Alice'} to {name: string; age?: number} is valid because age is optional.

3. Final Answer:

   let person: {name: string; age?: number} = {name: 'Alice'}; -> Option B

4. Quick Check:

   Optional properties can be omitted [OK]

Hint: Optional properties can be left out in assignments [OK]

Common Mistakes:

• Assigning object missing required properties
• Confusing optional and required properties
• Assigning extra properties to narrower types

3. What will be the output of the following TypeScript code?

interface A { x: number; }
interface B { x: number; y: number; }

let a: A = { x: 10 };
let b: B = { x: 10, y: 20 };

// Assignment
let c: A = b;
console.log(c.x);

medium

A. 10

B. 20

C. undefined

D. Compilation error

Solution

1. Step 1: Understand assignment compatibility from B to A

   Type B has all properties of A plus extra. Assigning B to A is allowed because A's shape is a subset of B's.

2. Step 2: Check the value of c.x

   Since b.x is 10, c.x will be 10 after assignment.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Subset shape assignment = Allowed and value preserved [OK]

Hint: Assign wider type to narrower type if subset matches [OK]

Common Mistakes:

• Expecting error when assigning wider to narrower type
• Confusing property values with property names
• Thinking extra properties cause errors

4. Identify the error in this TypeScript code snippet:

interface Person { name: string; age: number; }

let p: Person = { name: 'Bob' };

medium

A. Missing property 'age' in the object assigned to 'p'

B. Property 'name' should be a number, not a string

C. Interface 'Person' cannot be used as a type

D. No error, code is valid

Solution

1. Step 1: Check required properties in interface

   The interface requires both name and age properties.

2. Step 2: Verify object assigned to variable

   The object only has name, missing age, causing a type error.

3. Final Answer:

   Missing property 'age' in the object assigned to 'p' -> Option A

4. Quick Check:

   All required properties must be present [OK]

Hint: Check all required properties are present in assigned object [OK]

Common Mistakes:

• Ignoring missing required properties
• Assuming optional properties are required
• Thinking interface cannot be used as type

5. Given these interfaces:

interface X { a: number; b: string; }
interface Y { a: number; b: string; c?: boolean; }

Which assignment is valid and why?

let x: X = { a: 1, b: 'hello' };
let y: Y = { a: 1, b: 'hello', c: true };

// Assignments
x = y;
y = x;

hard

A. Both assignments are valid because optional properties don't matter

B. Only y = x is valid because X has fewer properties

C. Only x = y is valid because Y has all properties of X plus optional ones

D. Both assignments are invalid due to property mismatch

Solution

1. Step 1: Analyze x = y assignment

   Y has all properties of X plus an optional c. Assigning Y to X is allowed because X's shape is a subset of Y's.

2. Step 2: Analyze y = x assignment

   X lacks property c which Y expects optionally. Assigning X to Y is not allowed because c might be missing and is optional but not guaranteed.

3. Step 3: Reconsider optional property assignment rules

   Assigning X to Y is invalid because Y expects c optionally, but the source object does not have it, and TypeScript does not allow missing optional properties when assigning to a type that includes them.

4. Final Answer:

   Only x = y is valid because Y has all properties of X plus optional ones -> Option C

5. Quick Check:

   Optional properties allow assignment from wider to narrower types only [OK]

Hint: Optional properties allow assignment from wider to narrower types only [OK]

Common Mistakes:

• Thinking optional properties block assignment both ways
• Assuming extra properties cause errors always
• Confusing required and optional properties