试图在javascript中理解.some（）

Question

This is a portion of code from https://bl.ocks.org/mbostock/4183330

I am trying to understand how names.some(function(d)...}) works.

1. Shouldn't the anonymous function passed to .some() return a conditional statement that "countires" can be evaluated against?

2. When will names.some(..) return true or false?

3. Why doesn't d.name = n.name create "name" property in "countries" without "return"?

queue()
    .defer(d3.json, "world-110m.json")
    .defer(d3.tsv, "world-country-names.tsv")
    .await(ready);

function ready(error, world, names) {
    var countries = topojson.feature(world,world.objects.countries).features
    countries = countries.filter(function(d) {
        return names.some(function(n) {
                  if (d.id == n.id) return d.name = n.name;
     });
})

Answer 1

1) Since it's using names.some() , the function is testing each element of names , not countries .

2) When any of the names have an id that matches d.id and n.name is not empty.

3) It will always create the property. But if there's no return , .some() doesn't get a truth value that it can evaluate.

It would probably be easier to understand if they'd written:

return names.some(function(n) {
    if (d.id == n.id) {
        d.name = n.name;
        return d.name;
    } else {
        return false;
    }
});

return d.name = n.name; combines the assignment and return value into a single statement. And the code is making use of the fact that functions implicitly return undefined when they don't execute a return statement, and undefined is falsey.