Javascript链接方法在一起

Question

Lets say I have an array with multiple movies, these movie objects contain things like title/rating/etc...

if I do something like so:

let filteredMapList = arr.filter(function(obj){
  return obj.Rating > 8.0;
}).map(function(obj){
  return {title: obj.Title,rating: obj.Rating};
});

This should work and only return movies that are above an 8.0 and return only the Title and Rating in a new object.

So my confusion is...which one is exactly running first? Is map running first and then going through filter or is filter running first and then running map (Which seems odd....even though thats how it's laid out).

When chaining methods together, how do you exactly....follow the chain. Also does the obj thats getting passed between methods need to be the same? If not....then im really confused how it actually works.

Answer 1

They chain and run exactly in the order they are in. It's a bit more obvious if you rewrite it like this:

let a = arr.filter(function(obj){
  return obj.Rating > 8.0;
});

let b = a.map(function(obj){
  return {title: obj.Title,rating: obj.Rating};
});

The filter happens first and goes to a , then map is called on a and goes to b with your result.

Chaining functions is the same, you're just skipping the step where you create a variable for each step.

Answer 2

filter runs first, map second. map receives as an argument = what filter returned.