如何找到在 function 中声明的参数？

Question

I'm working on the web speech api for my own projects. I'm confused by this function:

colors.forEach(function(v, i, a){
  console.log(v, i);
  colorHTML += '<span style="background-color:' + v + ';"> ' + v + ' </span>';
});

I understand that v, i & a are parameters, but I can't see where they are declared. They are used in the function and I want to understand how they are used.

Many thanks

Edit: parameters within the parentheses

Answer 1

How do I find parameters declared in a function?

By reading the documentation .

but I can't see where they are declared

They are declared right there in the function expression that creates the function you pass as a callback.

They get values when the function is called.

 arr.forEach(callback(currentValue [, index [, array]])[, thisArg])

So v is the current value being looped over, i is its index in the array, and a is the array itself.