Functions in JavaScript serve multiple purposes. They allow you to group related code together, reuse code in different parts of a program, and pass data between different parts of a program.

To define a function in JavaScript, you use the function keyword followed by the function name and parentheses. Inside the parentheses, you specify the parameters that the function will receive. The function body, where the code that will be executed when the function is called is placed, is enclosed in curly braces.

To call a function in JavaScript, you simply use the function name followed by parentheses. You can pass arguments to the function by placing them inside the parentheses when you call the function.

Function declarations are hoisted to the top of their scope, while function expressions are not. Function expressions can be used as values, while function declarations cannot. Function declarations can be named, while function expressions cannot.

The this keyword in JavaScript refers to the current object. The value of this depends on how the function is called. When a function is called as a method of an object, this refers to that object. When a function is called as a standalone function, this refers to the global object.

Arguments are the values that are passed to a function when it is called. Parameters are the variables that are declared in the function definition. When a function is called, the arguments are assigned to the parameters.

A callback function is a function that is passed as an argument to another function. The callback function is then called from within the other function. Callback functions are often used to handle asynchronous operations.

Synchronous functions execute immediately, meaning that the code inside the function will be executed before the code after the function call. Asynchronous functions execute after some time, meaning that the code inside the function will be executed after the code after the function call.

A higher-order function is a function that takes another function as an argument, returns another function, or both. Higher-order functions are often used to create reusable code and to abstract away complex logic.

The bind() method in JavaScript creates a new function that, when called, has its this keyword set to the provided value. This is useful for creating functions that can be called with a specific context.

The apply() method in JavaScript calls a function with a given this value and arguments provided as an array. This is useful for calling functions with a specific context and arguments.

The call() method in JavaScript calls a function with a given this value and arguments provided as individual parameters. This is useful for calling functions with a specific context and arguments.

The bind() method creates a new function that, when called, has its this keyword set to the provided value. The apply() and call() methods call an existing function with a given this value and arguments.

A closure in JavaScript is a function that has access to the variables of its outer scope, even after the outer scope has finished executing. This is because the function retains a reference to the outer scope's variables.

Global variables are declared outside of any function, while local variables are declared inside a function. Global variables are accessible from anywhere in the program, while local variables are only accessible from within the function in which they are declared. Global variables can be reassigned, while local variables cannot.