Merge pull request #692 from Chalarangelo/glossary

[ENHANCEMENT] Glossary

glossary/Cross-site-scripting-xss.md

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+### Cross-site scripting (XSS)
+
+XSS refers to client-side code injection where the attacker injects malicious scripts into a legitimate website or web application. 
+This is often achieved when the application does not validate user input and freely injects dynamic HTML content.

glossary/DOM.md

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+### DOM
+
+The DOM (Document Object Model) is a cross-platform API that treats HTML and XML documents as a tree structure consisting of nodes. 
+These nodes (such as elements and text nodes) are objects that can be programmatically manipulated and any visible changes made to them are reflected live in the document. 
+In a browser, this API is available to JavaScript where DOM nodes can be manipulated to change their styles, contents, placement in the document, or interacted with through event listeners.

glossary/Functional-programming.md

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+### Functional programming
+
+Functional programming is a paradigm in which programs are built in a declarative manner using pure functions that avoid shared state and mutable data. 
+Functions that always return the same value for the same input and don't produce side effects are the pillar of functional programming.

glossary/Promise.md

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+### Promise
+
+The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
+A Promise can be in one of these states: pending(initial state, neither fulfilled nor rejected), fulfilled(operation completed successfully), rejected(operation failed).

glossary/Recursion.md

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+### Recursion
+
+Recursion is the repeated application of a process. 
+In JavaScript, recursion involves functions that call themselves repeatedly until they reach a base condition. 
+The base condition breaks out of the recursion loop because otherwise the function would call itself indefinitely. 
+Recursion is very useful when working with nested data, especially when the nesting depth is dynamically defined or unkown.