Node, Reading from JSON file returns an empty array

Question

function readTextFile(filename, callback){
    var fs = require('fs');
    fs.readFile(filename, 'utf8', function(err, data){
        if(err){
            throw err;
        }else{
            callback(data);
        }
    });
}

var set1 = [];
var set2 = [];
var set3 = [];
var set4 = [];
var set5 = [];

readTextFile("./data/names.json", function(text){
    var names = JSON.parse(text);
    set1 = names.prop1.prop2;
    set2 = names.prop1.prop3;
    set3 = names.prop1.prop4;
    set4 = names.prop5.prop2;
    set5 = names.prop5.prop3;
});
console.log(set1);

Running this section of code, using "node jsonRead.js" returns [ ] for any array called in console.log(). This section of code used to work about 80% of the time, but now it doesn't work at all, and I'm not sure why. I would appreciate any advice on how to get it working again.

I think this has been answered here - stackoverflow.com/questions/10058814/get-data-from-fs-readfile — CD-jS
– CD-jS, Commented May 19, 2016 at 15:51
isn't that asynchronous code? readTextFile() returns immediately, which means set1 will not yet have been populated. — Marc B
– Marc B, Commented May 19, 2016 at 15:52
Why is my variable unaltered after I modify it inside of a function? — Jonathan Lonowski
– Jonathan Lonowski, Commented May 19, 2016 at 16:12
Possible duplicate of Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference — Heretic Monkey
– Heretic Monkey, Commented Oct 25, 2018 at 12:00

chresse · Accepted Answer · 2016-05-19 15:51:25Z

1

maybe the read process is still going on.

try the console.log(set1); in the callback:

readTextFile("./data/names.json", function(text){
    var names = JSON.parse(text);
    set1 = names.prop1.prop2;
    set2 = names.prop1.prop3;
    set3 = names.prop1.prop4;
    set4 = names.prop5.prop2;
    set5 = names.prop5.prop3;

    console.log(set1);
});

This is cause the callbackfunction of readTextFile is executed when reading finished, whereas the other code is executed as written.

You could solve this by using Promises. Checkout the q library for example: https://github.com/kriskowal/q

answered May 19, 2016 at 15:51

chresse

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Comments

Sumeet Kumar Yadav · Accepted Answer · 2016-05-19 15:53:19Z

1

Node.js paradigm of executing code is Asynchronous , means it will execute next line of code after calling async method . In your above example readTextFile is called and execution will continue to next line . It will not wait for return .

answered May 19, 2016 at 15:53

Sumeet Kumar Yadav

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1 Comment

Calvin D. Over a year ago

How can I make the function return the data I need exactly when I need it, while still keeping it asynchronous?

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