Flattening an Array With Tests

gistfile1.js

class MyArray {
	constructor() {
		this.flattenedArray = [];
	}

	flattenArray(array) {
		let me = this;

		array.forEach(function(element) {
			if (!Array.isArray(element)) {
				me.flattenedArray.push(element);
			} else {
				me.flattenArray(element);
			}
		});
	}
}

// specs code
describe("My Array Tests", function() {
	let unflattenedArray = new MyArray();
	let arrayToChange = [[1,2,[3]],4];
   
	it("Should initialize the class", function() {
		expect(unflattenedArray.flattenedArray).toEqual([]);
	});

	it("Should flatten [[1,2,[3]],4]", function() {
		unflattenedArray.flattenArray([[1,2,[3]],4]);
		expect(unflattenedArray.flattenedArray).toEqual([1,2,3,4]);
	});
});


// load jasmine htmlReporter
(function() {
  var env = jasmine.getEnv();
  env.addReporter(new jasmine.HtmlReporter());
  env.execute();
}());

It can bee tested here:
https://jsfiddle.net/1by0jokq/3/