javascript回调-使用fs.readFile处理读取文件

Question

For a project, I am using the net module to create a 'mini web framework'

I'm having a lot of trouble dealing with this one callback

var sendFile(path) {
  fs.readFile(path, config, this.handleRead.bind(this));
}

where readFile is defined as:

var handleRead = function(contentType, data, err) {
  if (err) {
    console.log(err);         //returns properly
  } else {
    console.log(data);        //returns properly
    console.log(contentType)  //returning undefined
}

So far this code works in the sense that I can catch errors and also write the data properly.

My question is : how do I send the contentType through the call back?

I've tried -

var sendFile(path) {
  var contentType = ContentType['the path type']
  fs.readFile(path, config, this.handleRead(contentType).bind(this));
}

But then this causes data and err to be undefined.

I'm quite new to js and am still confused about how to work with callbacks. Any input is appreciated!

Answer 1

.bind() lets you do more than just set the "context" ( this value of a function). You can also "bind" arguments in the function.

Try:

function sendFile(path) {
  var contentType = ContentType['the path type']
  fs.readFile(path, config, this.handleRead.bind(this, contentType));
}

This will pass a callback with its context set to whatever this is and its 1st parameter set to contentType . As long as this callback is called with data (and possibly err ), then everything will work.