A closure in JavaScript is a feature that allows inner functions to access the outer scope of a function. It is created automatically whenever a function is created, and it helps in binding a function to its outer boundary. A closure is the combination of a function bundled together with references to its surrounding state, which is the lexical environment. In other words, a closure gives you access to an outer functions scope from an inner function. Closures are useful because they let you associate data (the lexical environment) with a function that operates on that data.

Some key points about closures in JavaScript include:

• Closures are created every time a function is created, at function creation time.
• A closure can have free variables together with an environment that binds those variables.
• A closure is a record storing a function together with an environment, which is a mapping associating each free variable of the function with the value or reference to which the name was bound when the closure was created.
• Closures can be used to create private variables in JavaScript.
• Closures can capture variables in block scopes and module scopes as well.

In summary, a closure in JavaScript is a powerful feature that allows inner functions to access the outer scope of a function, and it is created automatically whenever a function is created. Closures are useful for creating private variables and maintaining state between events in event-driven programming.