A closure is a function that has access to variables in its outer (enclosing) lexical scope, even after the outer function has returned. It's often used for data privacy, creating function factories, and in module patterns.

The 'this' keyword refers to the object it belongs to. Its value depends on the context in which it is used. In a method, 'this' refers to the owner object. In a function, 'this' refers to the global object (in non-strict mode) or undefined (in strict mode). In an event, 'this' refers to the element that received the event.

Prototypal inheritance is a method by which an object can inherit properties and methods from another object. In JavaScript, each object has an internal link to another object called its prototype. When trying to access a property of an object, the property will be sought on the object, then its prototype, then the prototype's prototype, and so on.

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It has three states: pending, fulfilled, or rejected. Promises are used to handle asynchronous operations in a more manageable way, avoiding callback hell and providing better error handling.

Async/await is syntactic sugar built on top of Promises. An async function returns a Promise, and the await keyword is used to pause the execution of the function until the Promise is resolved or rejected, making asynchronous code look and behave more like synchronous code.

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase. This means that variables can be used before they are declared, but only their declarations are hoisted, not their initializations.

The event loop is a mechanism that allows JavaScript to perform non-blocking operations by offloading operations to the system kernel whenever possible. It continuously checks the call stack and the task queue. If the call stack is empty, it takes the first event from the queue and pushes it to the call stack, effectively allowing asynchronous code to be executed.

Type coercion is the automatic or implicit conversion of values from one data type to another (such as strings to numbers). JavaScript uses type coercion in various operations, such as when using the `==` operator, which can lead to unexpected results.

JavaScript modules are reusable pieces of code that can be exported from one program and imported for use in another program. Modules allow for better code organization and reuse. ES6 introduced the `import` and `export` keywords to define modules.

Memory leaks occur when memory that is no longer needed is not released. In JavaScript, this can happen due to unintended references that prevent garbage collection. Common causes include global variables, closures, and event listeners. Prevention techniques include nullifying references, using weak references, and properly removing event listeners.

Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions. Key concepts include pure functions, immutability, first-class functions, higher-order functions, and function composition.

The JavaScript engine is a program that executes JavaScript code. Examples include V8 (used in Chrome and Node.js) and SpiderMonkey (used in Firefox). The runtime provides the environment in which JavaScript code is executed, including the call stack, memory heap, and APIs for interacting with the system.

Error handling in JavaScript is done using `try…catch` blocks. Code that may throw an error is placed inside the `try` block, and the `catch` block contains code to handle the error. The `finally` block can be used to execute code after the `try` and `catch` blocks, regardless of whether an error occurred.

Callbacks are functions passed as arguments to other functions to be executed later. Promises represent the eventual completion or failure of an asynchronous operation and provide methods like `then` and `catch` for handling results. Async/await is syntactic sugar on top of Promises, allowing asynchronous code to be written in a synchronous style.

Best practices include using `const` and `let` instead of `var`, avoiding global variables, using strict mode, writing modular code, handling errors properly, using descriptive variable and function names, and following consistent coding conventions and style guides.