如何从.map（）JS创建不可变的映射

Question

So I have a function like this

myNewMap = oldMap.map((check) => {
     if (check.get('_id') === against.get('_id')) return check;
});

The only problem is myNewMap is the same size as the oldMap (which makes sense), but is there a way to stop this, or do I need a different approach?

Answer 1

I think you want filter

myNewMap = oldMap.filter((check) => {
   return check.get('_id') === against.get('_id');
});

Or even shorter:

myNewMap = oldMap.filter(check => check.get('_id') === against.get('_id'));

This will return a new Map with only the values that meet the predicate.

Answer 2

After you map over the array you can do

Object.seal( yourMappedArrayObject );

That will make that object then immutable.

Object.seal