What if you need to retrieve data using a DOM node rather than a property name? A JavaScript `Map` stores key–value pairs and accepts keys of any datatype. Unlike a plain object, it is directly iterable. This is the `Map` data structure, not the array method `Array.prototype.map`. section 25.0
Create and use a Map
`const values = new Map([["width", 400], [3, "three"]]);` `values.set("height", 90);` `values.get("height"); // 90` `values.get("missing"); // undefined` Create an empty Map with `new Map()`, or pass an array of `[key, value]` pairs. Passing a plain object in place of those pairs does not work. Here, the string and number keys remain distinct; `set` adds a pair, and `get` retrieves its value. section 25.1 section 25.2
A Map’s entries are not its object properties: `Object.keys(values)` does not reveal its stored keys. Use `values.keys()` or `values.values()` to obtain iterators, and spread an iterator into an array when needed. Maps also offer `has`, `delete`, `clear`, `entries`, `forEach`, and `size`. section 25.2
Choose the container to match the job:
- Use a plain object for straightforward named options such as `width` and `height`; its properties support object-style destructuring. section 25.3
- Use a Map when you need non-string keys or want to iterate key–value pairs in a guaranteed order. Map destructuring is array-style and follows position, not key name. section 25.3
- Keep a reference to an object key when looking it up. Two separately created arrays with the same contents do not serve as the same key. section 25.3
Render search facets
Store facets as pairs, then visit each name and value directly: `for (const [name, value] of facets) {` ` // render name and value` `}` With a plain object, the loop instead obtains property names through `Object.keys(facets)` and looks up each value separately. section 25.3
Listing 26.4. src/cards.js
A card object serves as the key, while a boolean records whether that particular card is flipped. The flip functions update the value through `set`. section 26.1
For object-keyed data that should not keep its keys from being garbage-collected, consider `WeakMap` instead. Unlike `Map`, it is not iterable. For practice, write functions that copy a Map with its entries sorted by key or value, then invert its keys and values; assume unique values for the inversion. section 25.5
