Map Set WeakMap WeakSet

map-set-weakmap-weakset.js

// Today I learned about some intracicies of Map, Set, WeakMap and WeakSet

/**
  Map
  1. Maps can have non-string keys, even objects! Any value is valid, including NaN.
  2. When iterating over Maps, insertion order is preserved for iteration.
  3. Construct a Map from an object using Object.entries(obj). This converts an object to Map's expected format of nested arrays.
  4. Has .keys(), .values(), and .entries() methods. 
*/


const map = new Map([['b', 2], ['a', 1], ['c', 3]]);

map.forEach((value, key, map) => {
  console.log(key);
  console.log(value);
});

/**
  Set
  1. Setifying something is a good way to remove duplicates in an array of values.
  2. When iterating over sets, insertion order is also preserved. 
    - If a value is encountered twice, the initial insert defines the order in which it is returned during iteration.
  3. Constructing a set can be done from a simple array.
  4. Also has .keys(), .values(), and .entries() methods. Returns iterables. 
*/

const set = new Set([1, 2, 3])

set.forEach((value, value2, set) => {
  console.log(value);
  console.log(value2);
});

/**
  WeakMap and WeakSet
  Like Map and Set except:
  1. WeakMap keys MUST be object references
  2. WeakSet values MUST be object references
  3. WeakMap keys / weakSet values can be garbage collected if no other references point to them.
    - this means they can be deleted from the weakSet / WeakMap automatically.
    
  This might be useful if you have some contextual information about an object that should only live for the lifetime of the object.
  For example, if a user logs in, and you keep track of the user object in session storage, you might store some lifetime metadata
  about the user in a weakMap. 
  
  Because keys and values can be garbage collected at any time WeakMap and WeakSet do not have size() methods.
*/