Pass additional parameter to Javascript callback function [duplicate]

Question

This question already has answers here:

Pass an extra argument to a callback function (5 answers)

Closed 9 years ago.

I need to watch a small number of directories in a Node.JS application:

function updated(event, filename){
 log("CHANGED\t/share/channels/" + filename);
}
for(i in channels)
 fs.watch('share/channels/' + channels[i], {persistent: false}, updated);

The problem is that fs.watch only passes the filename to the callback function, without including the directory it's in. Is there a way I can somehow pass in an extra parameter to the updated() function so it knows where the file is?

I think I'm looking for something similar to Python's functools.partial, if that helps any.

Answer 1

Example using JS Bind

Doc: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind

Tip, the bound parameters occur before the call-time parameters.

my_hello = 'Hello!'
my_world = {
 'antartica': 'cold',
}
anonymous_callback = function (injected1, injected2, param1, param2) {
 param1 = param1 ? param1 : 'One';
 param2 = param2 ? param2 : 'Two';
 console.log('param1: (' + typeof(param1) + ') ' + param1)
 console.log('param2: (' + typeof(param2) + ') ' + param2)
 console.log('injected1: (' + typeof(injected1) + ') ' + injected1)
 console.log('injected2: (' + typeof(injected2) + ') ' + injected2)
 console.log(injected2)
}.bind(this, my_hello, my_world)
anonymous_callback('Param 1', 'Param 2')

Output:

param1: (string) Param 1
param2: (string) Param 2
injected1: (string) Hello!
injected2: (object) [object Object]
{ antartica: 'cold' }

• bitfidget: Thank you, I've been battling with this issue for hours and your explanation is so clear that I was able to finally make it work.

Answer 2

You can pass a different function for each iteration:

var getUpdatedFunction = function(folderName) {
 return function(event, filename) {
 log("CHANGED\t" + folderName + "/" + filename);
 };
};
for(i in channels) {
 foldername = 'share/channels/' + channels[i];
 fs.watch(foldername, {persistent: false}, getUpdatedFunction(foldername));
}

Answer 3

You can use Function.bind:

function updated(extraInformation, event, filename) {
 log("CHANGED\t/share/channels/" + extraInformation + filename);
}
for(i in channels)
 fs.watch('share/channels/' + channels[i], {persistent: false},
 updated.bind(null, 'wherever/it/is/'));

• ThorSummoner: I don't follow your example at all. Did you even explicitly reference the extra parameter you added to the function?
• Ry-: @ThorSummoner: updated.bind(null, 'wherever/it/is/') returns a function that can take two arguments, prepend 'wherever/it/is/, and then pass that to updated (with null being the unused value for this).
• Proximo: This answer was heaven sent.

Answer 4

You can pass additional callback in place

function updated(channel, filename) {
 log('CHANGED\t ' + channel + '/' + filename);
}
for(i in channels) {
 channel = '/share/channels/' + channels[i];
 fs.watch(channel, {persistent: false}, function (extra, event, filename) {
 updated(channel, filename);
 });
}