package.json

{
  "name": "@derpixler/jest-prototype-tests",
  "version": "1.0.0",
  "description": "",
  "main": "the-prototype.js",
  "scripts": {
    "test": "jest"
  },
  "jest": {
    "verbose": true,
    "setupFiles": []
  },
  "author": {
      "name": "René Reimann",
      "email": "info@rene-reimann.de",
      "url": "http://www.rene-reimann.de"
    },
  "license": "MIT",
  "devDependencies": {
    "eslint": "^5.16.0",
    "eslint-config-google": "^0.13.0",
    "jest": "^24.8.0"
  }
}

prototype.js

var myFunction = function(arg1, arg2) {
	this.options = {};
	this.testNodes = [];
	this.options.arg1 = arg1;
	this.options.arg2 = arg2;

	this.fooBar();
	this.testFunction();
};

myFunction.prototype = {
	testFunction: function(){
		this.options.arg3 = 'abc';
	},
};

myFunction.prototype.fooBar = function(){
		const nodes = document.querySelectorAll('body span');

		nodes.forEach(node => {
			this.testNodes.push(node.innerHTML);
		});
};

module.exports = myFunction;

prototype.test.js

var myFunction = require('./prototype');

beforeEach( () => {
	document.body.innerHTML =
		'<div>' +
		'  <span id="username">FOObar</span>' +
		'  <button id="button" />' +
		'</div>';

	this.myFunc = new myFunction();
} );

describe('Prototype Test', () => {
	test( 'call', () => {
		const testNodes = this.myFunc.testNodes;

		expect(Array.isArray(testNodes) && testNodes.length > 0).toBe(true);

		testNodes.map(node =>{
			expect(node).toBe('FOObar');
		});

	} );
});