map

2021.12.22
Map in JavaScript
#JavaScript#basics
use client /components/post/reExport ,
  date:  ,
  tags: [ ],
  imgUrl:  ,
  desc:  ,
  body: (
    <>
      <p>Map is is a special type collection, like an object with keys of any data type.</p>

      <H>Object vs Map</H>

      <Code block jsx>{  };
      let obj = {};
      obj[john] = 123;
      obj // {[object Object]: 123}

      let map = new Map(); 
      map.set(john, 123); 
      map // [key: {name:  }, value: 123]
       new Map([iterable]) 
      let map = new Map() // creates the map
      // or
      let map = new Map([
        [ ],
        [1,     ],
        [true,  ]
      ]);
       map.set(key, value) 
      map.set( ) // a string key
      map.set(1,  ) // a numeric key
      map.set(true,  ) // a boolean key
       map.get(key) }</Code> - works, but isn’t the right way
          </li>
        </ul>
      </p>

      <Code block jsx>{ 
      map.get(1); //  }</Code>

      <H>Size</H>

      <Code block jsx>{ }</Code>

      <Hs>Delete</Hs>

      <ul>
        <li>
          <Code>{ }</Code> - removes the value by the key
        </li>
        <li>
          <Code>{ }</Code> - removes everything from the map
        </li>
      </ul>

      <H>Has</H>

      <ul>
        <li>
          <Code>{ }</Code> - returns true if the key exists, false otherwise
        </li>
      </ul>

      <H>Iteration</H>

      <p>Iteration goes in the same order as the values were inserted</p>

      <Code block jsx>{ , 500],
        [ , 350],
        [ , 50]
      ]);
  
      // returns iterable of keys, values & entries
      recipeMap.keys() // MapIterator { }
      recipeMap.values() // MapIterator{500, 350, 50}
      recipeMap.entries() // MapIterator{  => 50} // it’s used by default in for..of.
  
      for (let key of recipeMap.keys()) alert(key) // cucumber, tomatoes, onion
      for (let value of recipeMap.values()) alert(value) // 500, 350, 50
      for (let entry of recipeMap) alert(entry) // cucumber,500 (and so on) // the same as of recipeMap.entries()
      
      recipeMap.forEach( (value, key, map) => {
        alert(\ ) // cucumber: 500 etc
      })
       
        map.set( ).set(true,  )
        map // Map(3){ , true =>  }</Code>

      <H>{ }</H>

      <Code block jsx>{ ],
        [1,     ],
        [true,  ]
      ]);
  
      map.get( }</Code>

      <H>{ }</H>

      <Code block jsx>{ ,
        age: 30
      };
      
      let map = new Map(Object.entries(obj));
      map.get( }</Code>

      <H>{ }</H>

      <Code block jsx>{ , 1],
        [ , 2],
      ])
      prices // { banana: 1, orange: 2, meat: 4 }
       
      let map = new Map();
      let obj1 = {name:  }
      let obj2 = {name:  }
      map.set(obj1,  );
      map.set(obj2,  );
      obj1 = null
      obj2 = null
      map.size // 2 // bad

      let weakMap = new WeakMap();
      let obj1 = {name:  }
      let obj2 = {name:  }
      weakMap.set(obj1,  );
      weakMap.set(obj2,  );
      obj1 = null
      obj2 = null
      weakMap.size // undefined
      // When obj gets garbage collected, weakMap will be removed as well   
       
        // example from work where we get data from db, put it into Map and then check something against it

          const rowsFromDb = await q.execute()

          type AppId = { hpi: boolean; hsi: boolean }
          type RegisterAppIdMap = Map<number, AppId>

          const registerAppIdMap = rowsFromDb.reduce<RegisterAppIdMap>((map, row) => {
            const key = row.RegisterId

            const current = map.get(key)

            if (current === undefined) {
              map.set(key, {
                hpi: false,
                hsi: false,
              })
            } else {
              if (row.AppId ===  ) {
                current.hpi = true
              }

              if (row.AppId ===  ) {
                current.hsi = true
              }
            }

            return map
          }, new Map())

          // then pass registerAppIdMap further
      