Examine the code in the editor. It's using a method that has side effects in the program, causing incorrect behaviour

JavaScript
1

Tell us what’s happening:
i dont understand what is happening in this code and how to start solving it. i will appreciate the solution so i can continue to functional programming

Your code so far


// tabs is an array of titles of each site open within the window
var Window = function(tabs) {
 this.tabs = tabs; // We keep a record of the array inside the object
};

// When you join two windows into one window
Window.prototype.join = function (otherWindow) {
 this.tabs = this.tabs.concat(otherWindow.tabs);
 return this;
};

// When you open a new tab at the end
Window.prototype.tabOpen = function (tab) {
 this.tabs.push('new tab'); // Let's open a new tab for now
 return this;
};

// When you close a tab
Window.prototype.tabClose = function (index) {

 // Only change code below this line

 var tabsBeforeIndex = this.tabs.splice(0, index); // Get the tabs before the tab
 var tabsAfterIndex = this.tabs.splice(index + 1); // Get the tabs after the tab

 this.tabs = tabsBeforeIndex.concat(tabsAfterIndex); // Join them together

 // Only change code above this line

 return this;
};

// Let's create three browser windows
var workWindow = new Window(['GMail', 'Inbox', 'Work mail', 'Docs', 'freeCodeCamp']); // Your mailbox, drive, and other work sites
var socialWindow = new Window(['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium']); // Social sites
var videoWindow = new Window(['Netflix', 'YouTube', 'Vimeo', 'Vine']); // Entertainment sites

// Now perform the tab opening, closing, and other operations
var finalTabs = socialWindow
 .tabOpen() // Open a new tab for cat memes
 .join(videoWindow.tabClose(2)) // Close third tab in video window, and join
 .join(workWindow.tabClose(1).tabOpen());
console.log(finalTabs.tabs);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.193 Safari/537.36.

Challenge: Understand the Hazards of Using Imperative Code

Link to the challenge:

2

copy the code and paste it in a tool like this: http://pythontutor.com/javascript.html#mode=edit

if you follow the execution, you will notice that something weird happens at one point, and you need to change something to make sure that instead it is the wanted behaviour that happens