Pass additional parameter to callback function

Question

I have a pretty simple function that is reading a file with HTML 5 FileReader:

var reader = new FileReader();
var currentFileType = file.type;
reader.onloadend = readCompleted;
reader.onerror = fail;

reader.readAsArrayBuffer(file);

and the readCompleted callback function looks like this:

    function readCompleted(evt) {
        if (evt.target.readyState != FileReader.DONE) {
            return;
        }

        // The binary data is the result.
        var requestData = evt.target.result;

        // SOME MORE CODE HERE
    }

Thing is that 'evt' parameter is passed by default. How can I pass one additional parameter to this callback function in order to have currentFileType variable available? I want this function signature to look like this: function readCompleted(evt, fileType) {//code here} and pass somehow currentFileType to it.

arcyqwerty · Accepted Answer · 2016-03-01 18:25:06Z

2

You can use Function.bind to bind an argument to the function before assigning it to onloadend.

For example:

var a = function() { console.log(arguments); }
var b = a.bind(null, 123)
b('abc') // -> [123, "abc"]

In your case, it would be:

reader.onloadend = readCompleted.bind(null, file.type);

and readCompleted should have the following signature:

function readCompleted(fileType, evt) { ... }

edited Mar 1, 2016 at 18:25

answered Mar 1, 2016 at 18:23

arcyqwerty

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1 Comment

martinedwards Over a year ago

In 15 years of development, I've never known that the default parameters are still passed, but at the end of the bind! You're my hero.

Mike Cluck · Accepted Answer · 2016-03-01 18:23:57Z

1

Just assign a different function to reader.onloadend.

reader.onloadend = function(evt) {
    readCompleted(evt, currentFileType);
};

answered Mar 1, 2016 at 18:23

Mike Cluck

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Comments

Sathik Khan · Accepted Answer · 2016-03-01 18:28:04Z

0

You can write like this,

var reader = new FileReader();
var currentFileType = file.type;
reader.onloadend = function(evt){
    readCompleted(evt,currentFileType);
};
reader.onerror = fail;

The readCompleted will look like this,

function readCompleted(evt,ft) { //code... }

answered Mar 1, 2016 at 18:28

Sathik Khan

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