Functional Programming: Understand the Hazards of Using Imperative Code (I am having some issues here)

JavaScript
1

Tell us what’s happening:
I have gone through my code I dont know where my mistake is
It keeps giving me an error
Help me look through it

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) {
 var tabsBeforeIndex = this.tabs.splice(0, index); // get the tabs before the tab
 var tabsAfterIndex = this.tabs.splice(index); // get the tabs after the tab

 this.tabs = tabsBeforeIndex.concat(tabsAfterIndex); // join them together 
 return this;
};

// Let's create three browser windows
var workWindow = new Window([ 'GMail', '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' , '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);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36 Edg/87.0.664.66.

Challenge: Understand the Hazards of Using Imperative Code

Link to the challenge: