How structural typing differs from nominal typing in Typescript - Visual Walkthrough

Concept Flow - How structural typing differs from nominal typing

Start: Define Type A

↓

Define Type B

↓

Compare Types

↓

Nominal Typing: Compare names

↓

Structural Typing: Compare shape

↓

Assign or Reject

↓

End

This flow shows how nominal typing compares type names, while structural typing compares the shape (properties) of types to decide compatibility.

Execution Sample

Typescript

type Cat = { name: string };
type Dog = { name: string };

let cat: Cat = { name: "Whiskers" };
let dog: Dog = cat; // Allowed in structural typing

Assigning a Cat object to a Dog variable works because both have the same shape in structural typing.

Execution Table

Step	Action	Type A	Type B	Comparison	Result
1	Define Type Cat	{ name: string }			Type Cat created
2	Define Type Dog		{ name: string }		Type Dog created
3	Create cat object	cat = { name: "Whiskers" }			cat assigned
4	Assign cat to dog	Cat	Dog	Compare names (Nominal): Cat != Dog	Nominal: Error
5	Assign cat to dog	Cat	Dog	Compare shape (Structural): same properties	Structural: Allowed
6	End				Assignment allowed in structural typing

💡 Nominal typing rejects assignment due to different names; structural typing allows it due to matching shape.

Variable Tracker

Variable	Start	After Assignment	Final
cat	undefined	{ name: "Whiskers" }	{ name: "Whiskers" }
dog	undefined	Error in nominal typing	{ name: "Whiskers" } in structural typing

Key Moments - 2 Insights

Why does assigning 'cat' to 'dog' cause an error in nominal typing but not in structural typing?

Nominal typing checks if the type names match exactly (Cat vs Dog), so it rejects the assignment (see execution_table step 4). Structural typing checks if the properties match, ignoring names, so it allows it (step 5).

What does 'shape' mean in structural typing?

Shape means the properties and their types inside the object. If two types have the same properties with the same types, they have the same shape (execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, at which step does nominal typing reject the assignment?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Check the 'Result' column for nominal typing rejection.

According to the variable tracker, what is the value of 'dog' after assignment in structural typing?

A undefined

B Error

C { name: "Whiskers" }

D null

💡 Hint

Look at the 'Final' column for 'dog' in variable_tracker.

If Type Dog had an extra property, how would structural typing handle assigning 'cat' to 'dog'?

A Allow assignment because names differ

B Reject assignment because shapes differ

C Allow assignment ignoring extra properties

D Error due to missing name property

💡 Hint

Structural typing requires matching properties; extra properties cause mismatch.

Concept Snapshot

Nominal typing compares type names exactly.
Structural typing compares object shapes (properties).
In TypeScript, structural typing allows assignment if shapes match.
Nominal typing rejects if names differ, even if shapes match.
Structural typing is more flexible and common in TypeScript.

Full Transcript

This visual execution shows how nominal and structural typing differ in TypeScript. We define two types, Cat and Dog, both with a name property. When assigning a Cat object to a Dog variable, nominal typing checks the type names and rejects the assignment because Cat and Dog are different names. Structural typing compares the properties inside the types and allows the assignment because both have the same shape: a name string. The variable tracker shows that 'cat' holds the object with name 'Whiskers', and 'dog' gets the same object in structural typing but causes an error in nominal typing. Key moments clarify why nominal typing rejects based on names and what shape means in structural typing. The quiz tests understanding of these steps and concepts.