how to create a scientific calculator with html, css and linking it with javascript to make it work
Firstly, welcome to the forums.
While we are primarily here to help people with their Free Code Camp progress, we are open to people on other paths, too. Some of what you are asking is pretty trivial in the Free Code Camp context, so you might find that if you’re not getting the instruction and material you need in your current studies, the FCC curriculum will really help you get started. At a modest guess I’d say investing a 4-5 hours working through the curriculum here will really pay off. You can find the curriculum at https://learn.freecodecamp.org.
With your current questions, we don’t have enough context to know what you already know or don’t know, so it is impossible to guide you without just telling you the answer (which we won’t do).
It is pretty typical on here for people to share a codepen / jsfiddle example of what they have tried so that anyone helping has more of an idea of what help is actually helpful.
Please provide some example of what you’ve tried and I’m sure you’ll get more help.
Happy coding 
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Good morning sir @DanCouper, and thanks for your prompt reply.
Actually, I have worked on creating a user interface of a simple calculator using css and html codes but could not link them with javascript codes in order to make the calculator function. I am learning the basics of javascript for the first time and do not no much yet. Please it will be helpful to get an assistance on this platform.
Share the code that you have so far so that we know where exactly you are struggling.
As for adding JavaScript functionality, you are going to want to use the getElement(s)By functions to get DOM elements (for example, document.getElementById()). Once you have the element (which, by the way, you can set to a variable like var a = document.getElementById(“the_id”)), then you can interact with its numerous properties