Unsure why a jest test is failing - node.js

Question

I have the following test code. The test is failing, but I'm not entirely sure why (I have a suspicion, but I'm unsure of how to fix it). Here are the high level classes:
// file myFile.js
const EventEmitter = require('events').EventEmitter;
const em = new EventEmitter();
class Foo {
constructor () {
this.support = 0;
em.on('onNewValue', this._handleNewValue.bind(this));
}
_handleNewValue (newValue) {
this.support = newValue;
console.log(this.support); // Prints whatever newValue is
}
}
class Bar {
setValue (newValue) {
em.emit('onNewValue', newValue);
}
}
Here's the test
const foo = new Foo();
expect(foo.support).toEqual(0); // Passes - initial value of support is 0
const bar = new Bar();
bar.setValue(10); // Emits an event that causes foo to set it's internal support value to 10.
console.log('Checking support'); // This prints after the console.log in foo._handleNewValue.
expect(foo.support).toEqual(10); // This fails - still reports initial value of 0
The high-level idea is that bar.setValue() emits an event that is listened to by the Foo class. The event listener updates the value of it's internal member support to be what the event payload is.
The test fails and reports the initial value of support. My hunch is that there is some timing issue going on (the expect gets called first, and then the event listener inside the Foo class).
However, if I console.log, the log in the event listener prints first, and then the log for 'Checking support', which does not seem to go along that theory.
Could someone help me figure out why this test is failing, and how to fix it? I don't want to explicitly call foo._handleNewValue in the test.

Answer 1

I tried your code using jest and it worked perfectly. Here's what I've tried
// file myFile.js
const EventEmitter = require('events').EventEmitter;
const em = new EventEmitter();
class Foo {
constructor() {
this.support = 0;
em.on('onNewValue', this._handleNewValue.bind(this));
}
_handleNewValue(newValue) {
this.support = newValue;
console.log(this.support); // Prints whatever newValue is
}
}
class Bar {
setValue(newValue) {
em.emit('onNewValue', newValue);
}
}
test('should catch bar emitted event', () => {
const foo = new Foo();
expect(foo.support).toBe(0)
const bar = new Bar()
bar.setValue(10)
expect(foo.support).toBe(10)
})