Duck typing mental model in TypeScript - Time & Space Complexity

Time Complexity: Duck typing mental model in TypeScript

O(n)

Understanding Time Complexity

We want to see how the time it takes to check if an object fits a type grows as the object gets bigger.

How does TypeScript's duck typing affect the time it takes to verify an object's shape?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

interface Bird {
  fly(): void;
  sing(): void;
}

function canFly(bird: any): bird is Bird {
  return typeof bird.fly === 'function' && typeof bird.sing === 'function';
}

const obj = {
  fly() { console.log('Flying'); },
  sing() { console.log('Singing'); },
  walk() { console.log('Walking'); }
};

canFly(obj);
    

This code checks if an object has the methods needed to be considered a Bird by looking at its properties.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking each required property on the object.
• How many times: Once per property checked (here, two properties: fly and sing).

How Execution Grows With Input

As the number of properties to check grows, the time to verify grows linearly.

Input Size (number of properties)	Approx. Operations
2	2 checks
10	10 checks
100	100 checks

Pattern observation: The time grows directly with the number of properties checked.

Final Time Complexity

Time Complexity: O(n)

This means the time to check the object's shape grows in a straight line as the number of properties to check increases.

Common Mistake

[X] Wrong: "Checking one property means constant time no matter how many properties the object has."

[OK] Correct: Even if the object is large, you must check each required property one by one, so more properties mean more checks and more time.

Interview Connect

Understanding how duck typing checks scale helps you explain how TypeScript verifies types behind the scenes, showing your grasp of both coding and performance.

Self-Check

"What if we added nested objects to check inside the main object? How would the time complexity change?"

Practice

1. What does duck typing mean in TypeScript?

easy

A. Only objects created from the same class are compatible.

B. If an object has the required properties or methods, it is accepted regardless of its explicit type.

C. TypeScript requires explicit class inheritance to match types.

D. TypeScript checks the runtime type of objects strictly.

Solution

1. Step 1: Understand duck typing concept

   Duck typing means an object is accepted if it has the needed properties or methods, regardless of its declared type.

2. Step 2: Compare with other options

   The other options describe strict or class-based typing, which is not how TypeScript works.

3. Final Answer:

   If an object has the required properties or methods, it is accepted regardless of its explicit type. -> Option B

4. Quick Check:

   Duck typing = structural compatibility [OK]

Hint: Focus on properties, not explicit types, for duck typing [OK]

Common Mistakes:

• Thinking TypeScript requires class inheritance
• Confusing runtime type checks with compile-time checks
• Assuming exact type names must match

2. Which of the following is a correct TypeScript syntax to define a type that matches any object with a name string property and a quack() method returning void?

easy

A. type Duck = { name: string; quack: () => void; };

B. interface Duck { name: string; quack: void; }

C. type Duck = { name: string; quack(): string; };

D. interface Duck { name: string; quack: () => string; }

Solution

1. Step 1: Check property and method types

   The name property must be a string, and quack must be a method returning void.

2. Step 2: Validate each option

   type Duck = { name: string; quack: () => void; }; correctly defines quack as a function returning void. The other options have incorrect method signatures or return types.

3. Final Answer:

   type Duck = { name: string; quack: () => void; }; -> Option A

4. Quick Check:

   Correct method signature = type Duck = { name: string; quack: () => void; }; [OK]

Hint: Method type must be a function signature with correct return type [OK]

Common Mistakes:

• Using method return type as property type
• Confusing void and string return types
• Incorrect interface method syntax

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

interface Duck { name: string; quack: () => void; }

function makeItQuack(duck: Duck) {
  duck.quack();
}

const obj = { name: 'Daffy', quack: () => console.log('Quack!'), swim: () => console.log('Swim') };
makeItQuack(obj);

medium

A. No output

B. Swim

C. Error: obj does not match Duck interface

D. Quack!

Solution

1. Step 1: Check if obj matches Duck interface

   obj has name (string) and quack (function returning void), so it matches by duck typing.

2. Step 2: Call makeItQuack with obj

   Calling duck.quack() runs console.log('Quack!'), printing "Quack!".

3. Final Answer:

   Quack! -> Option D

4. Quick Check:

   Duck typing allows extra properties, output = Quack! [OK]

Hint: Extra properties don't block duck typing; focus on required ones [OK]

Common Mistakes:

• Thinking extra properties cause errors
• Expecting 'Swim' output instead of 'Quack!'
• Assuming runtime type errors

4. Identify the error in this TypeScript code using duck typing:

interface Duck { name: string; quack: () => void; }

const notADuck = { name: 'Donald' };

function makeItQuack(duck: Duck) {
  duck.quack();
}

makeItQuack(notADuck);

medium

A. notADuck is missing the quack method required by Duck interface.

B. The name property should be a number, not a string.

C. makeItQuack should accept any object without type restrictions.

D. There is no error; code runs fine.

Solution

1. Step 1: Check properties of notADuck

   notADuck has name but lacks quack method required by Duck interface.

2. Step 2: Understand TypeScript type checking

   TypeScript will report an error because notADuck does not satisfy the Duck type.

3. Final Answer:

   notADuck is missing the quack method required by Duck interface. -> Option A

4. Quick Check:

   Missing required method = Type error [OK]

Hint: Check all required properties/methods exist for the type [OK]

Common Mistakes:

• Ignoring missing methods in objects
• Assuming extra properties cause errors
• Thinking TypeScript allows missing required members

5. Given this code, which object will TypeScript accept as a valid Duck type?

interface Duck { name: string; quack: () => void; }

const a = { name: 'Daisy', quack: () => console.log('Quack!') };
const b = { name: 'Goose', quack: 'loud' };
const c = { name: 'Swan', quack: () => 'quack' };
const d = { name: 'Mallard' };

function testDuck(duck: Duck) { duck.quack(); }

Which call to testDuck will NOT cause a TypeScript error?

hard

A. testDuck(c);

B. testDuck(b);

C. testDuck(a);

D. testDuck(d);

Solution

1. Step 1: Check each object's quack property type

   a.quack is a function returning void (correct). b.quack is a string (wrong). c.quack is a function returning string (wrong return type). d.quack is missing (wrong).

2. Step 2: Determine which matches Duck interface

   Only object a matches the Duck interface exactly.

3. Final Answer:

   testDuck(a); -> Option C

4. Quick Check:

   Correct method signature only in a [OK]

Hint: Method must be function returning void to match Duck [OK]

Common Mistakes:

• Accepting string or wrong return type as method
• Ignoring missing methods
• Confusing method return types