在JavaScript中，forEach回调如何在后台运行？ 有收益声明吗？

Question

In javascript, I realize that the callback function is passed each element of the collection in a forEach method call, but how does it have access to it? In Ruby, we yield to the block.

def calculation(a, b)
  yield(a, b)
end

puts calculation(5, 6) { |a, b| a + b } # addition
puts calculation(5, 6) { |a, b| a - b } # subtraction

But in Javascript, how does the function get passed each element of the array? What's going on behind the scenes with the built in forEach function?

fruits.forEach(function (item, index, array) {
  console.log(item, index);
});

Answer 1

Well, it's not that hard to implement more easily than its done in the reference :

function forEach(fn) {
    for (var i = 0; i < this.length; ++i) { // 'this' is the array, of course
        fn(this[i], i, this);
    }
}

_{I've also removed thisArg for simplicity's sake}

The actual implementations do more checking and are more robust, but this illustrates the idea, I think. I can imagine it being implemented natively, as well.