Interpretation of the implementation principles of JS’s built-in iterable objects from a source code perspective

Interpretation of the implementation principles of JS built-in iterable objects from a source code perspective

In JavaScript, many built-in objects are iterable, which means we can use loop structures to iterate over their elements. For example, arrays, strings, and Maps are all iterable objects. This article will explain the implementation principles of JavaScript's built-in iterable objects from a source code perspective, and provide specific code examples.

The implementation principle of JavaScript's built-in iterable objects mainly involves two aspects: iterators and iterable protocols.

1. Iterator (Iterator): An iterator is an object that provides a next() method for traversing the elements in an iterable object. Each time the next() method is called, the iterator will return an object containing the value and done attributes, where value represents the value of the current element, and done represents whether the traversal has ended.

Let us take an array as an example to see how the iterator is implemented:

const arr = [1, 2, 3];
const iterator = arr[Symbol.iterator]();

console.log(iterator.next()); // {value: 1, done: false}
console.log(iterator.next()); // {value: 2, done: false}
console.log(iterator.next()); // {value: 3, done: false}
console.log(iterator.next()); // {value: undefined, done: true}

In the above example, we obtain it by calling the Symbol.iterator method of the array object. An iterator object. Then, by calling the next() method continuously, we can iterate through the elements in the array. The traversal ends when the done attribute is true.

2. Iterable Protocol: The Iterable Protocol is a specification that stipulates that iterable objects must have a Symbol.iterator method, and the method must return an iterator object.

The following is an example of a custom iterable object:

const myIterableObject = {
  [Symbol.iterator]() {
    let count = 1;

    return {
      next() {
        if (count <= 3) {
          return { value: count++, done: false };
        } else {
          return { value: undefined, done: true };
        }
      }
    };
  }
};

for (const item of myIterableObject) {
  console.log(item);
}
// 输出：1, 2, 3

In the above example, the myIterableObject object implements the Symbol.iterator method and returns an iterator object. The next() method is implemented in the iterator object, and each call returns the current value and traversal status. When traversing the myIterableObject object through a for...of loop, the corresponding iterator object will be automatically called for traversal.

In fact, iterator and iterable protocols are a design pattern in JavaScript and are widely used in many scenarios. For example, Generators are also based on implementations of iterators and iterable protocols.

In summary, the implementation principle of JavaScript's built-in iterable objects is implemented through iterators and iterable protocols. The iterator object provides the next() method to traverse the elements in the iterable object, and the iterable protocol stipulates that the iterable object must have the Symbol.iterator method and return the iterator object. By flexibly using iterators and iterable protocols, we can customize iterable objects and achieve more iteration functions.

I hope this article can help you gain a deeper understanding of the implementation principles of JavaScript's built-in iterable objects.

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