An iterator is an object whose `next()` method returns an iteration result: an object with `value` and `done` properties. An iterable provides a `Symbol.iterator` method that returns an iterator; the two protocols work together but are not the same. section 9.1 section 9.1.2
This protocol lets JavaScript consumers request values without depending on a collection’s internal structure.
- A result with `done: false` supplies another value; `done: true` signals the end. Results that break the contract cause a `TypeError`. section 9.1.2
- Custom objects can expose iteration through `Symbol.iterator`, enabling consumers such as `for...of` and the spread operator to use them. section 9.1.3
- An iterator need not be iterable: a `next()` method alone supports direct requests, while adding `Symbol.iterator` enables `for...of`. section 9.1.3
Listing 9.1 Random-number generator using an iterator
function randomNumberIterator(size = 1) {
function nextRandomInteger(min) { #1
return function(max) {
return Math.floor(Math.random() * (max - min)) + min;
};
}
const numbers = Array(size) #2
.fill(1)
.map(min => nextRandomInteger(min)(Number.MAX_SAFE_INTEGER));
return {
next() {
if(numbers.length === 0) {
return {done: true}; #3
}
return {value: numbers.shift(), done: false}; #4
}
};
}
let it = randomNumberIterator(3);
console.log(it.next().value); // 1334873261721158 #5
console.log(it.next().value); // 6969972402572387 #5
console.log(it.next().value); // 3915714888608040 #5
console.log(it.next().done); // true
A random-number iterator shows `next()` returning values until the sequence is exhausted. section 9.1.3
class Block {
index = 0;
constructor(index, previousHash, data = [], difficulty = 0) {
this.index = index;
this.previousHash = previousHash;
this.data = data;
this.nonce = 0;
this.difficulty = difficulty;
this.timestamp = Date.now();
this.hash = this.calculateHash();
}
//...
[Symbol.iterator]() {
return this.data[Symbol.iterator]();
}
}
A `Block` delegates iteration to its stored data, making its transactions available through the iterator protocol. section 9.1.3
