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 - http://stackoverflow.com/questions/10058814/get-data-from-fs-readfile — CD-jS, May 19 '16 at 15:51
isn't that asynchronous code? readTextFile() returns immediately, which means `set1` will not yet have been populated. — Marc B, May 19 '16 at 15:52
[Why is my variable unaltered after I modify it inside of a function?](https://stackoverflow.com/questions/23667086/why-is-my-variable-unaltered-after-i-modify-it-inside-of-a-function-asynchron) — Jonathan Lonowski, May 19 '16 at 16:12
Possible duplicate of [Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference](https://stackoverflow.com/questions/23667086/why-is-my-variable-unaltered-after-i-modify-it-inside-of-a-function-asynchron) — Heretic Monkey, Oct 25 '18 at 12:00

score 1 · Answer 1 · answered May 19 '16 at 15:51

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

score 1 · Answer 2 · answered May 19 '16 at 15:53

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 '16 at 15:53

Sumeet Kumar Yadav

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How can I make the function return the data I need exactly when I need it, while still keeping it asynchronous? – Calvin D. May 19 '16 at 21:19

Node, Reading from JSON file returns an empty array

2 Answers2