Type-safe event emitter pattern in Typescript - Time & Space Complexity

Time Complexity: Type-safe event emitter pattern

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a type-safe event emitter grows as we add more events or listeners.

Specifically, how does the number of listeners affect the work done when an event is emitted?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

interface Events {
  message: (text: string) => void;
  error: (code: number) => void;
}

class EventEmitter {
  private listeners: { [K in keyof Events]?: Events[K][] } = {};

  on(event: K, listener: Events[K]) {
    (this.listeners[event] ??= []).push(listener);
  }

  emit(event: K, ...args: Parameters) {
    (this.listeners[event] ?? []).forEach(listener => listener(...args));
  }
}
    

This code defines a type-safe event emitter that stores listeners and calls them when an event is emitted.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The forEach loop that calls each listener when an event is emitted.
• How many times: It runs once for each listener registered for that event.

How Execution Grows With Input

When you emit an event, the time to run grows with the number of listeners for that event.

Input Size (listeners)	Approx. Operations (listener calls)
10	10
100	100
1000	1000

Pattern observation: The work grows directly with the number of listeners. More listeners mean more calls.

Final Time Complexity

Time Complexity: O(n)

This means the time to emit an event grows linearly with the number of listeners for that event.

Common Mistake

[X] Wrong: "Emitting an event always takes the same time, no matter how many listeners there are."

[OK] Correct: Each listener must be called, so more listeners mean more work and more time.

Interview Connect

Understanding how event emitters scale helps you write efficient code and explain your reasoning clearly in interviews.

Self-Check

"What if we changed the emitter to call only the first listener instead of all? How would the time complexity change?"

Practice

1. What is the main benefit of using a type-safe event emitter in TypeScript?

easy

A. It makes the code run faster by optimizing event handling.

B. It ensures events and their data match expected types, preventing runtime errors.

C. It automatically logs all emitted events to the console.

D. It allows emitting events without defining their types.

Solution

1. Step 1: Understand type safety in event emitters

   Type-safe event emitters use TypeScript types to ensure events and their data match expected types.

2. Step 2: Identify the benefit of type safety

   This prevents runtime errors by catching mismatches during development, improving code reliability.

3. Final Answer:

   It ensures events and their data match expected types, preventing runtime errors. -> Option B

4. Quick Check:

   Type safety = prevents runtime errors [OK]

Hint: Remember: type-safe means matching event names and data types [OK]

Common Mistakes:

• Thinking it improves runtime speed
• Assuming it logs events automatically
• Believing it allows untyped events

2. Which of the following is the correct way to define an interface for a type-safe event emitter with events login (payload: string) and logout (no payload)?

easy

A. interface Events { login: string; logout: null; }

B. interface Events { login: string; logout: void; }

C. interface Events { login: string; logout: undefined; }

D. interface Events { login: string; logout: never; }

Solution

1. Step 1: Understand event payload types

   Events with no payload should use undefined to indicate no data is expected.

2. Step 2: Check interface options

   interface Events { login: string; logout: undefined; } uses logout: undefined, which correctly represents no payload for the logout event.

3. Final Answer:

   interface Events { login: string; logout: undefined; } -> Option C

4. Quick Check:

   No payload event = undefined type [OK]

Hint: Use 'undefined' type for events with no payload [OK]

Common Mistakes:

• Using void which is a function return type
• Using null which means absence of value
• Using never which means impossible type

3. Given this type-safe event emitter interface and usage:

interface Events {
  message: string;
  error: { code: number; message: string };
}

const emitter = new TypedEmitter<Events>();
emitter.on('message', (msg) => console.log(msg));
emitter.emit('error', { code: 404, message: 'Not found' });

What will be the output when the error event is emitted?

medium

A. No output because no listener for 'error' event is registered.

B. Logs: { code: 404, message: 'Not found' }

C. Throws a type error at runtime.

D. Logs: 'Not found'

Solution

1. Step 1: Check registered event listeners

   Only a listener for 'message' event is registered, no listener for 'error'.

2. Step 2: Understand event emission behavior

   Emitting 'error' without a listener produces no output; no automatic logging occurs.

3. Final Answer:

   No output because no listener for 'error' event is registered. -> Option A

4. Quick Check:

   Emit without listener = no output [OK]

Hint: Only events with listeners produce output [OK]

Common Mistakes:

• Assuming emit logs automatically
• Expecting runtime error without listener
• Confusing event data with console output

4. Consider this code snippet using a type-safe event emitter:

interface Events {
  update: number;
}

const emitter = new TypedEmitter<Events>();
emitter.on('update', (data: string) => {
  console.log(data);
});

What is the problem with this code?

medium

A. There is no problem; the code is correct.

B. The event name 'update' is not defined in the interface.

C. The emitter instance is not created correctly.

D. The listener callback parameter type does not match the event payload type.

Solution

1. Step 1: Check event payload type in interface

   The 'update' event expects a number payload.

2. Step 2: Check listener callback parameter type

   The callback expects a string, which mismatches the expected number.

3. Final Answer:

   The listener callback parameter type does not match the event payload type. -> Option D

4. Quick Check:

   Listener param type must match event payload type [OK]

Hint: Match listener parameter type to event payload type [OK]

Common Mistakes:

• Ignoring type mismatch warnings
• Assuming any type is allowed
• Confusing event names with payload types

5. You want to create a type-safe event emitter for a chat app with events:
- join: payload is username (string)
- leave: payload is username (string)
- message: payload is an object with user (string) and text (string)

Which interface correctly defines these events for type safety?

hard

A. interface ChatEvents { join: string; leave: string; message: { user: string; text: string }; }

B. interface ChatEvents { join: string; leave: string; message: string; }

C. interface ChatEvents { join: { username: string }; leave: { username: string }; message: { user: string; text: string }; }

D. interface ChatEvents { join: string; leave: undefined; message: { user: string; text: string }; }

Solution

1. Step 1: Analyze event payload requirements

   'join' and 'leave' expect a username string; 'message' expects an object with user and text strings.

2. Step 2: Match interface to requirements

   interface ChatEvents { join: string; leave: string; message: { user: string; text: string }; } matches exactly: 'join' and 'leave' as string, 'message' as object with user and text.

3. Final Answer:

   interface ChatEvents { join: string; leave: string; message: { user: string; text: string }; } -> Option A

4. Quick Check:

   Match payload types exactly for type safety [OK]

Hint: Match event names and payload types exactly in interface [OK]

Common Mistakes:

• Using wrong payload types for events
• Omitting required object structure
• Using undefined for events that need payload