**Article Class: "_2rhmJa"** --- ### **What is an Iterator?** An **interface** that provides a unified access mechanism for various data structures. Any data structure that implements the `Iterator` interface can perform **traversal operations** (i.e., sequentially process all members of the data structure). --- ### **Functionality** - **Unifies access** for various data structures - **Allows sequential ordering** of data structure members - **ES6 introduces** a new traversal command `for...of` loop, with the `Iterator` interface primarily designed for consumption by `for...of` loops. --- ### **Default `Iterator` Interface** In **ES6**, the default `Iterator` interface is defined on the `Symbol.iterator` property. A data structure is considered **iterable** if it has the `Symbol.iterator` property. The `Symbol.iterator` property itself is a **function** that generates the default traversal pointer. When executed, it returns a **traversal pointer**. The property name `Symbol.iterator` is an **expression** returning a `Symbol` object's `iterator` attribute, a pre-defined value of type `Symbol`. --- ### **Features** - **Core Feature**: A **pointer object** with a `next` method. - Each call to `next` returns an **information object** containing `value` (current member's value) and `done` (boolean indicating if traversal is complete). - `value` is the value of the current member, and `done` is a boolean indicating whether traversal has ended. - **Additional Methods**: - `next` (required) - `return` (optional) - `throw` (optional) - **`return` method** returns an **object** (required by `Generator` syntax). - **Use cases**: - `for...of` loop to prematurely exit (e.g., due to error or `break` statement) - Resource cleanup or release before traversal completes - **`throw` method** is primarily used with `Generator` functions; typical traversal pointers do not use it. --- ### **Simple Implementation** Use a **`Generator` function**: ```javascript let obj = { * [Symbol.iterator]() { yield 'hello'; yield 'world'; } }; for (let x of obj) { console.log(x); } ``` --- ### **Built-in Types** - `Array` - `Map` - `Set` - `String` - `TypedArray` - `arguments` object of a function - `NodeList` object --- ### **Scenarios** - **Destructuring assignment**: Default call - **Spread operator (`...`)** - `yield*` - **Other contexts where arrays are used as parameters** - `for...of` - `Array.from()` - `Map()`, `Set()`, `WeakMap()`, `WeakSet()` (e.g., `new Map([['a', 1], ['b', 2]])`) - `Promise.all()` - `Promise.race()` --- ### **References** - [Symbol.iterator](https://links.jianshu.com/go?to=https%3A%2F%2Fdeveloper.mozilla.org%2Fzh-CN%2Fdocs%2FWeb%2FJavaScript%2FReference%2FGlobal_Objects%2FSymbol%2Fiterator) - [ES6 R入门 Iterator 和 for...of 循环](https://links.jianshu.com/go?to=https%3A%2F%2Fes6.ruanyifeng.com%2F%23docs%2Fiterator) --- **Note**: The article maintains its original structure, technical terminology, and formatting while ensuring clarity and professional tone.