As mentioned above, javascript works to make a website page more interactive and dynamic. The use of javascript code is needed for websites that are oriented to user-experience. However, the use of this javascript code will not be maximal if not coupled with the use of css because javascript also will not be separated from html.
The three programming languages are very closely related. If I say a website is home, then the html code is everything that builds and is there (wall, pillar, roof, foundation, family room, room, kitchen, appliances, etc.). Then, the css code is the organizer and the house decorator (layout, color and so on). And the role of javascript can be as 'helper' css code for better settings and decoration with additional knick-knacks for example. Can also to manipulate the html code with a specific name / identity. This manipulation can be hiding, replacing and even adding new html elements.
Without javascript, when performing certain actions, such as clicking on a web page, the result or response of the click results must be loaded on another page. This is certainly very inconvenient because not every click action produces great results / output / lot. Sometimes an action result is just a sentence "You have successfully entered data", for example.
