Mocha - Failed looping through array that gets initialized in before() - node.js

Question

Consider this test setup:
var expect = require('chai').expect;
var fs = require('fs');
describe('my test', function() {
var dataArr;
// get data
before(function(done) {
fs.readFile('myArray.json', 'utf8', function (err, data) {
dataArr = JSON.parse(data);
done();
});
});
// test data
dataArr.forEach(function(val, index) {
it('Testing element no ' + (index+1), function(done) {
expect(val).to.be.cool;
done();
});
});
});
This test returns the error Cannot read property 'forEach' of undefined because the loop is executed before before().
I found this approach on SO which uses nested its, but that seems to be a dirty workaround (googling mocha nested it proves that).
Isn't there any clean solution for this not so uncommon (?) problem?

Answer 1

Tests are defined synchronously. It is test function that is postponed by asynchronous done call, not dataArr.forEach.
In this case asynchronous readFile provides no benefits. It would be simpler with:
before(function() {
const data = fs.readFileSync('myArray.json', 'utf8');
dataArr = JSON.parse(data);
});
And there's no need for readFile at all. Reading the file isn't a part of a test and should be performed once. Node.js natively supports importing JSON files:
var expect = require('chai').expect;
var dataArr = require('./myArray.json');

Answer 2

You are executing the forEach method out of mocha's hands, you should move all your logic inside it method
So update your code like below:
it('Testing elements', function(done) {
expect(dataArr).all.to.be.cool;
});

Answer 3

I don't think any any cleaner solutions exist.
In your example, dataArr is undefined at the time it is evaluated, that is the member forEach doesn't yet exist because dataArr is not yet an array at that point. It would be defined inside the it()'s callback function scope as the it() callback function is called after the 'before' callback.
So the nested "describe-it" workaround works because it fixes the evaluation order such that dataArr is defined at that point.
So the already mentioned workaround is like this:
var assert = require('assert');
var fs = require('fs');
describe('my test', function() {
var dataArr;
// get data
before(function(done) {
dataArr = ["cool", "cool", "cool"]; // or fs.readFile or require
done();
});
// test it
it('test after data setup', function() {
describe('the tests', function() {
// now the dataArr is defined!
dataArr().forEach(function(val, index) {
it('Testing element no ' + (index+1), function(done) {
assert.equal(val, "cool");
done();
});
});
});
});
});
On the other hand if you don't alter the dataArr, you could evaluate it at once on spot of the declaration like:
var assert = require('assert');
var fs = require('fs');
describe('my test', function() {
const dataArr = ["cool", "cool", "cool"]; // or dataArr = require('testfile.json');
before(function(done) {
done();
});
// test it
// now the dataArr is defined!
dataArr().forEach(function(val, index) {
it('Testing element no ' + (index+1), function(done) {
assert.equal(val, "cool");
done();
});
});
});