如何确定nodejs/expressjs函数中可用的方法

Question

If I have a function like

request.get(options, function(req, res, body) {
    console.log(req, res, body)
});

Are the local functions variable names arbitrary (req,res,body). Like can they be anything? If that's the case, how does 'request' know how to interpret each function variable? Is it by the placement order.. as in the first req/whatever is always going to be a request? How do I know if there should be more than 3 local function variables and what they will do?

Answer 1

Are the local functions variable names arbitrary (req,res,body). Like can they be anything?

Yes, anything you like. (That said: Using consistent names makes it easy for people to understand your code. Using the standard names from the Express documentation makes it even easier.)

If that's the case, how does 'request' know how to interpret each function variable?

It doesn't. It calls the function you call it with arguments, like this:

// Hypothetical example of within the part of `request` that triggers the callback:
theCallback(theRequest, theResponse, theBody);

...and those arguments are bound to your parameters ( req , res , body ) by position : The first argument request passes your callback is bound to req , the second to res , and the third to body .

"Ah," you may be wondering, "but what if I don't define a body parameter and there are only two?" Doesn't matter; in JavaScript, it's fine if we call a function with more arguments than it has parameters (or fewer).

This is just like any other function call. The caller doesn't know the parameter name, just what arguments it passes and the order in which it should pass them.

For instance (see comments):

 // A fake "request" function function myRequest(callback) { // (Using a timer to make our callback happen later, just to more closely match `request`) setTimeout(function() { // Note that this doesn't know or care what the parameter // names are you've used in your callback callback("one", "two", "three"); }, 250); } // Your use of it myRequest(function(a, b, c) { // a, b, and c are bound by position, not name console.log(a, b, c); });

Answer 2

It does binding by order .

This means that this code:

request.get(options, function(req, res, next)...

req is a request because it appears first. The name you choose ( req , res ) have no impact on how the data is bound.

How do I know if there should be more than 3 local function variables and what they will do?

You either read the ExpressJS implementation code (you shouldn't) or you can read their docs or examples here .