Mastering Immutable.js

Merging maps by key

When merging maps, there's always the chance that keys will conflict. The question is, which value gets used? Let's illustrate this idea by merging two simple lists together:

const myMap1 = Map.of(  'one', 1, 'two', 2, 'three', 3);const myMap2 = Map.of(  'two', 22, 'three', 33, 'four', 4);const myMergedMap = myMap1.merge(myMap2);console.log('myMap1', myMap1.toJS());// -> myMap1 { one: 1, two: 2, three: 3 }console.log('myMap2', myMap2.toJS());// -> myMap2 { two: 22, three: 33, four: 4 }console.log('myMergedMap', myMergedMap.toJS());// -> myMergedMap { one: 1, two: 22, three: 33, four: 4 }

As you can see, myMergedMap contains every key from myMap1 and myMap2. There are two conflicting keys—two and three. By default, the