Iterators and generators are closely related and often used together, they are not the same. Let’s break it down clearly.
| Aspect | Iterator | Generator |
|---|---|---|
| Definition | Any object that implements the iterator protocol (__iter__() and __next__() in Python, or .next() in JavaScript). |
A special type of iterator that yields values using yield or yield*, often created with less boilerplate. |
| Creation | Manually implemented by defining a class (Python) or an object with .next() (JavaScript). |
Automatically created when a function contains yield. |
| Syntax | Verbose, needs boilerplate code. | Concise and easy to write. |
| Memory | Can be optimized if implemented carefully. | Always lazy (values are produced one at a time, on demand). |
| Use Case | When you need more control over the iteration protocol. | When you want to produce a sequence lazily with simpler syntax. |
🐍 Python Example
✅ Iterator (Manual)
class MyIterator:
def __init__(self, limit):
self.limit = limit
self.current = 0
def __iter__(self):
return self
def __next__(self):
if self.current >= self.limit:
raise StopIteration
val = self.current
self.current += 1
return val
it = MyIterator(3)
for val in it:
print(val) # 0 1 2
✅ Generator (Simpler!)
def my_generator(limit):
for i in range(limit):
yield i
for val in my_generator(3):
print(val) # 0 1 2
🌐 JavaScript Example
✅ Iterator (Manual)
const myIterator = {
current: 0,
limit: 3,
[Symbol.iterator]() {
return {
current: this.current,
limit: this.limit,
next() {
if (this.current < this.limit) {
return { value: this.current++, done: false };
}
return { done: true };
}
};
}
};
for (const item of myIterator) {
console.log(item);
}
✅ Generator
function* myGenerator(limit) {
for (let i = 0; i < limit; i++) {
yield i;
}
}
for (const val of myGenerator(3)) {
console.log(val); // 0 1 2
}
You can also have async generator but you need to await the iterator.
async function* myGenerator(limit) {
for (let i = 0; i < limit; i++) {
yield i;
}
}
for await (const val of myGenerator(3)) {
console.log(val); // 0 1 2
}
✅ Summary
| Feature | Iterator | Generator |
|---|---|---|
| How it’s made | Manually via class or object | Automatically via function* or yield
|
| Verbosity | More code | Less code |
| Use cases | Custom iteration logic | Simple, lazy sequence generation |
| Is lazy? | Can be | Always lazy |
🧠 Rule of Thumb:
Use generators when you want quick, lazy iteration.
Use iterators when you need more control or are implementing custom container types.
