Mastering Immutable.js

Lazy zipping

The zipping methods that we've looked at so far have all been called on lists. If you want to lazily zip lists together, you have to use a sequence, as follows:

names  .toSeq()  .zipWith(    (...maps) => Map().merge(...maps),    roles,    ages  )  .forEach(v => console.log('lazy zipWith', v.toJS()));  // -> lazy zipWith { name: 'Jeremy', role: 'Engineer', age: 34 }  // -> lazy zipWith { name: 'Garry', role: 'Designer', age: 23 }  // -> lazy zipWith { name: 'Katie', role: 'Programmer', age: 36 }

You don't have to convert each of your lists to sequences in order to lazy zip them together. By calling toSeq() on the names list just before calling zipWith(), you're zipping together one map at a time as it's fed into the forEach() side-effect. Now ...