使用fs.readfile.bind（context，pathArgument）时如何调用fs.readfile中的回调

Question

How does the callback in fs.readfile get called when using fs.readfile.bind(context,pathArgument) like so. //understandable because my task function knows the name of the callback parameter

async.series([function(callback){
//operation done callback()},...],finalCallback(err,result));

BUT //not understandable

async.series([fs.someOperation.bind(null,firstArgument),...],finalCallback(err,esult)) 

I believe I understand partial application;however, it would look something like this. function(callback){ fs.someOperation(firstArgument, ????)}(asyncCallbackFunc) and then I have no idea how the second argument is called... Thx, in advance for helping me clear this up.

Answer 1

All bind does is set the context of the callback. It is still a regular callback like any other. Except, it is explicitly told what this will be. Looks like in your case, it is set to null .

Answer 2

The bind function on function object allows you to set the context ie the value of this inside the function body as well as allow you to create a partial function in case you pass some arguments while calling bind .

For example:

function add(a,b) { 
  console.log(this);  
  return a+b; 
}

var newAdd = add.bind("hello world", 10);

The newAdd will be one argument function which gets added to 10 and the result is returned. Also when newAdd is called the "hello world" will be logged in console.

Now when your code says fs.readFile.bind(null, path) it means that the return function will be of one argument which is the callback for readfile ie the return function will be of form function(callback) { ... } which is exactly what is required to be passed to async.series

The main idea in the code you posted is to create a partial function that accepts only callback so that it can be passed to async.series the null argument doesn't play any role but you need to pass a context argument to call bind function, hence a null is passed as context arg.