That’s why Map and Set also exist.… Methods and properties are:
new Map() – creates the map.
map.set(key, value) – stores the value by the… map[key] isn’t the right way to use a Map
Although map[key] also works, e.g. we can set map… So we should use map methods: set, get and so on.
Map can also use objects as keys.… That’s what Map needs.
Built-in classes like Array, Map and others are extendable also.… Built-in methods like filter, map and others – return new objects of exactly the inherited type PowerArray… exists, it should return the constructor that JavaScript will use internally to create new entities in map… If we’d like built-in methods like map or filter to return regular arrays, we can return Array in Symbol.species… Other collections work similarly
Other collections, such as Map and Set, work alike.
Similar to that, if we use an object as the key in a regular Map… , then while the Map exists, that object exists as well.… Compare it with the regular Map example above.… The information is stored in a map: a user object is the key and the visit count is the value.… If we replace Map with WeakMap, then this problem disappears.
They are supported for:
Map
Set
Array
Plain objects also support similar methods, but the syntax is… Please note the distinctions (compared to map for example):
Map
Object
Call syntax
map.keys()… Transforming objects.Objects lack many methods that exist for arrays, e.g. map, filter and others.… Use array methods on that array, e.g. map, to transform these key/value pairs.
For each URL generated by URL.createObjectURL the browser stores a URL → Blob mapping… While there’s a mapping for a Blob, the Blob itself resides in the memory.… The mapping is automatically cleared on document unload, so Blob objects are freed then.… URL.revokeObjectURL(url) removes the reference from the internal mapping, thus allowing the Blob to be… After the revocation, as the mapping is removed, the URL doesn’t work any more.
The similar code for a Map… The pattern on the left side may be more complex and specify the mapping… We can also use more complex destructuring with nested objects and colon mappings… Destructuring assignment allows for instantly mapping an object or array onto many variables.… Object properties that have no mapping are copied to the rest object.
The native Map takes single value only as the key.… There are many solutions possible:
Implement a new (or use a third-party) map-like data structure that… Use nested maps: cache.set(min) will be a Map that stores the pair (max, result).… In our particular case we can just use a string "min,max" as the Map key.
Response headers.The response headers are available in a Map-like headers object in response.headers.… It’s not exactly a Map, but it has similar methods to get individual headers by name or iterate over… response.status – HTTP code of the response,
response.ok – true if the status is 200-299.
response.headers – Map-like
Existing data structures are not quite suitable for these purposes:
Using Map to create associations… names and images, or vice versa, will keep the image objects in memory since they are present in the Map… For this purpose, we can use a Map collection, whose values are WeakRef instances referring to the large… In short, we use a Map with string keys and WeakRef objects as their values.… Now we have a Map collection, where the keys – are image names as strings, and values – are WeakRef objects
Built-in objects: Internal slots.Many built-in objects, for example Map, Set, Date, Promise and others… For instance, Map stores items in the internal slot [[MapData]].… Internally, a Map… Now it works fine, because get trap binds function properties, such as map.set, to the target object (map… Unlike the previous example, the value of this inside proxy.set(...) will be not proxy, but the original map
When we need to iterate and return the data for each element – we can use map.… Most methods support “thisArg”.Almost all array methods that call functions – like find, filter, map,… To transform the array:
map(func) – creates a new array from results of calling func for every element… The function fn is called on each element of the array similar to map.
the parameter,
sort() – sort parameters by name, rarely needed,
…and it’s also iterable, similar to Map
The full syntax for Array.from also allows us to provide an optional “mapping
First, we can just switch to using Map for storage instead of plain objects, then everything’s fine:
A common trick is to map an array of job data into an array of promises, and then wrap that into Promise.all
But the attribute-property mapping is not one-to-one!
As you can see, it’s like a Map collection (setItem/getItem/removeItem), but also allows access by index
value
A value to encode.
replacer
Array of properties to encode or a mapping
Same letters in different layouts may map to different physical keys, leading to different codes.