mikemaccana mikemaccana - 2 months ago 8
Node.js Question

Using async module to fire a callback once all files are read

I'm using caolan's 'async' module to open an array of filenames (in this case, template file names).

Per the documentation, I'm using async.forEach(),so I can fire a callback once all operations have completed.

A simple test case is:

var async = require('async')
var fs = require('fs')

file_names = ['one','two','three'] // all these files actually exist

async.forEach(file_names,
function(file_name) {
console.log(file_name)
fs.readFile(file_name, function(error, data) {
if ( error) {
console.log('oh no file missing')
return error
} else {
console.log('woo '+file_name+' found')
}
})
}, function(error) {
if ( error) {
console.log('oh no errors!')
} else {
console.log('YAAAAAAY')
}
}
)


The output is as follows:

one
two
three
woo one found
woo two found
woo three found


I.e, it seems the final callback isn't firing. What do I need to do to make the final callback fire?

Answer

The function that is being run across all items must take a callback, and pass its results to the callback. See below (I've also separated fileName to improve readability):

var async = require('async')
var fs = require('fs')

var fileNames= ['one','two','three']


// This callback was missing in the question.
var readAFile = function(fileName, callback) {
    console.log(fileName)
    fs.readFile(fileName, function(error, data) {
        if ( error) {   
            console.log('oh no file missing')   
            return callback(error)
        } else {
            console.log('woo '+fileName+' found')
            return callback()
        }       
    })
}

async.forEach(fileNames, readAFile, function(error) {
    if ( error) {   
        console.log('oh no errors!')
    } else {
        console.log('YAAAAAAY')
    }
})

Returns:

one
two
three
woo one found
woo two found
woo three found
YAAAAAAY