Question

I’m trying to read a json structure into a global variable, but cannot seem to get it to work. I am using a callback for processing once it’s read from the file (that part is working).

I’d like to have “source_files” populated.

var fs = require('fs');
var source_files = [];

function readConfig(callback) { 
    fs.readFile('data.json', 'utf-8', function (err, content) {
        if (err) return callback(err);
        callback(content);
    });             
}                       

readConfig(function(config) { 
    var settings = JSON.parse(config);
    var inputs = settings.inputs;

    for (var id=0; id < inputs.length; id++) {
        source_files.push(inputs[id].replace('./',''));
    }           
});

console.log(source_files);

Answer 1

Remember that readFile is asynchronous. Your last line, console.log(source_files), will run before the readFile callback has been called, and thus before the readConfig callback is called. You need to move that into the readConfig callback.

With your code as-is, here’s what happens:

1. It creates the blank array.
2. It calls readConfig.
3. readConfig calls readFile.
4. readFile starts the asynchronous read operation and returns.
5. readConfig returns.
6. You log source_files, which is empty.
7. Later, the readFile operation finishes and the callback is called. It calls the readConfig callback.
8. The readConfig callback populates source_files, but it’s a bit like a tree falling in the forest at that point, as nothing observes that. 🙂