How structural typing differs from nominal typing in Typescript - Performance & Efficiency

Time Complexity: How structural typing differs from nominal typing

O(n)

Understanding Time Complexity

We want to understand how checking if two types match grows in cost as types get bigger or more complex.

How does the way TypeScript compares types affect the work it does?

Scenario Under Consideration

Analyze the time complexity of type compatibility checks in TypeScript's structural typing.

interface Point {
  x: number;
  y: number;
}

interface NamedPoint {
  x: number;
  y: number;
  name: string;
}

let p: Point = { x: 1, y: 2 };
const np: NamedPoint = { x: 1, y: 2, name: "A" };

// Structural typing allows this:
p = np;

This code shows TypeScript allowing assignment based on matching properties, not explicit names.

Identify Repeating Operations

When TypeScript checks if one type fits another, it compares each property.

• Primary operation: Comparing each property in the source and target types.
• How many times: Once for each property in the target type.

How Execution Grows With Input

As the number of properties in the types grows, the work to check compatibility grows too.

Input Size (n properties)	Approx. Operations
10	10 property comparisons
100	100 property comparisons
1000	1000 property comparisons

Pattern observation: The checking work grows directly with the number of properties.

Final Time Complexity

Time Complexity: O(n)

This means the time to check types grows in a straight line with the number of properties.

Common Mistake

[X] Wrong: "TypeScript compares types by their names, so checking is instant."

[OK] Correct: TypeScript compares the shape (properties) of types, so it must check each property, which takes time.

Interview Connect

Understanding how TypeScript checks types helps you explain how your code scales and why some checks take longer with bigger types.

Self-Check

What if TypeScript used nominal typing instead of structural typing? How would the time complexity of type checks change?

Practice

1. Which of the following best describes structural typing in TypeScript?

easy

A. Types are compatible if their properties and types match, regardless of their names.

B. Types must have the exact same name to be compatible.

C. Types are only compatible if they come from the same class.

D. Types are checked only at runtime for compatibility.

Solution

1. Step 1: Understand structural typing concept

   Structural typing means compatibility depends on the shape (properties and types) of objects, not their names.

2. Step 2: Compare with nominal typing

   Nominal typing requires exact type names or declarations to match, which is not how TypeScript works by default.

3. Final Answer:

   Types are compatible if their properties and types match, regardless of their names. -> Option A

4. Quick Check:

   Structural typing = shape-based compatibility [OK]

Hint: Think about shape, not names, for structural typing [OK]

Common Mistakes:

• Confusing structural typing with nominal typing
• Thinking type names must match for compatibility
• Assuming runtime checks for type compatibility

2. Which TypeScript code snippet correctly demonstrates structural typing compatibility?

easy

A. const a: {name: string} = {name: 'Alice'}; const b: {name: string} = a;

B. class A {} class B extends A {} const a: A = new B();

C. type A = {id: number}; type B = {name: string}; const a: A = {id: 1}; const b: B = a;

D. interface A {name: string;} interface B {name: number;} const a: A = {name: 'Bob'}; const b: B = a;

Solution

1. Step 1: Check property shapes for compatibility

   const a: {name: string} = {name: 'Alice'}; const b: {name: string} = a; assigns an object with a matching property name: string to another variable with the same shape, which is allowed.

2. Step 2: Analyze other options for incompatibility

   A mismatches properties (id vs name), B uses classes (nominal typing), D mismatches property types.

3. Final Answer:

   const a: {name: string} = {name: 'Alice'}; const b: {name: string} = a; -> Option A

4. Quick Check:

   Matching shapes = compatible [OK]

Hint: Match property names and types exactly for compatibility [OK]

Common Mistakes:

• Assuming different property names are compatible
• Ignoring property type mismatches
• Confusing class inheritance with structural typing

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

interface Point { x: number; y: number; }
interface Named { name: string; }

const p: Point = { x: 10, y: 20 };
const n: Named = { name: 'Origin' };

function printPoint(point: Point) {
  console.log(`x=${point.x}, y=${point.y}`);
}

printPoint(p);
printPoint(n as unknown as Point);

medium

A. x=10, y=20
x=0, y=0

B. x=10, y=20
x=undefined, y=undefined

C. x=10, y=20
x=Origin, y=undefined

D. Type error at compile time

Solution

1. Step 1: Understand structural typing and casting

   The object p matches Point shape, so printPoint(p) prints x=10, y=20. The object n does not have x or y, but is forced cast to Point.

2. Step 2: Predict output for forced cast

   At runtime, n lacks x and y, so accessing them prints undefined.

3. Final Answer:

   x=10, y=20 x=undefined, y=undefined -> Option B

4. Quick Check:

   Forced cast ignores shape, runtime undefined properties [OK]

Hint: Casting doesn't add missing properties, they stay undefined [OK]

Common Mistakes:

• Expecting runtime type errors
• Assuming cast adds missing properties
• Confusing property names with values

4. Identify the error in this TypeScript code related to structural typing:

interface A { id: number; }
interface B { id: number; name: string; }

const a: A = { id: 1 };
const b: B = a;

medium

A. Error because interfaces cannot be assigned to each other.

B. Error because a and b have different interface names.

C. No error, assignment is allowed due to structural typing.

D. Error because a lacks property name required by B.

Solution

1. Step 1: Check property requirements for assignment

   Variable b expects an object with id and name. Variable a only has id.

2. Step 2: Understand structural typing rules

   Structural typing requires the assigned object to have at least the required properties. Here, a is missing name, so assignment fails.

3. Final Answer:

   Error because a lacks property name required by B. -> Option D

4. Quick Check:

   Missing required properties cause assignment error [OK]

Hint: Check all required properties exist before assignment [OK]

Common Mistakes:

• Ignoring missing properties in assignment
• Thinking interface names must match for compatibility
• Assuming partial property match is enough

5. Given these TypeScript classes, which assignment is allowed due to structural typing?

class Person { name: string; constructor(name: string) { this.name = name; } }
class Employee { name: string; salary: number; constructor(name: string, salary: number) { this.name = name; this.salary = salary; } }

let p: Person = new Person('Alice');
let e: Employee = new Employee('Bob', 50000);

// Which assignment is valid?
// 1) p = e;
// 2) e = p;

hard

A. Both assignments are allowed due to structural typing.

B. Only assignment 2) e = p; is allowed because Person has all Employee properties.

C. Only assignment 1) p = e; is allowed because Employee has all Person properties.

D. Neither assignment is allowed because classes use nominal typing.

Solution

1. Step 1: Compare properties of Person and Employee

   Person has name. Employee has name and salary. Employee has all Person properties plus more.

2. Step 2: Apply structural typing rules for assignment

   Assigning e (Employee) to p (Person) is allowed because e has at least the properties of p. The reverse is not allowed because p lacks salary.

3. Final Answer:

   Only assignment 1) p = e; is allowed because Employee has all Person properties. -> Option C

4. Quick Check:

   Subtype with extra properties can assign to supertype [OK]

Hint: Assign subtype to supertype, not vice versa [OK]

Common Mistakes:

• Assuming both assignments are allowed
• Confusing nominal typing with structural typing here
• Ignoring extra properties in assignment compatibility