Maps
new Map([ ["key1", "val1"], ["key2", "val2"], ["key3", "val3"] ])
.set("key10", "val10")
.forEach(function(v,k,m) {
console.log(`key = ${k} value = ${v}`);
});Map
The Map object holds key-value pairs (entries) and remembers the original insertion order of the keys. It looks like an object’s properties, but
- does not contain any keys by default
- keys can be any value including function, not only String
- keys are stored in order of insertion
- propertysize allows to retrieve number of items
- a Map object is iterable
- a Map object is faster when items are frequently added or removed
| method | description |
|---|---|
| set(k, v) | Sets value for the key. Returns map object. |
| get(k) | Returns value associated with given key or undefined value if no element with this key. |
| has(k) | Returns true if entry with given key exist. |
| forEach(cb [, thisArg]) | Executes callback function for every entry. Callback functions has 3 parameters:
|
| entries() | Returns iterator for iterating over all entries (pair of key and value) of map. |
| values() | Returns iterator for iterating over all values of map in insertion order. |
| keys() | Returns iterator for iterating over all keys of map in insertion order. |
| clear() | Removes all entries from map. |
| delete(key) | Removes element with key. Returns false if element with given key does not exist. |
WeakMap
The WeakMap object is a collection of key/value pairs (entries) in which the keys are weakly referenced. So only objects allowed as a key, not primitive values. When key not used anywhere, entry will be removed by garbage collector. So you can't iterating over elements.
WeakMap has only 4 methods: delete(key), get(key), has(key), set(key, value). Property length is always 0 value.