In Java Script closure is a combination of a function and the lexical environment within which that function has declared. This environment consists of any local variables that were in scope at the time closure was created. Closures allow a function to access and manipulate variables from an outer scope even after the outer function has finished executing. This means that a closure can remember and access variables and parameters of its outer function, even after the outer function has returned.

A basic example to illustrate closures:

function outerfunction(){ let outervariable= 'out' ; function innerfunction(){ console.log(outervariable); } return innerfunction; } let closure=outerfunction(); closure();

In this example, outerfunction defines a variable outervariable and a function innerfunction that logs the value of outervariable. When outerfunction is called, it returns innerfunction, creating a closure. Even though outerfunction has finished executing, innerfunction still has access to the outervariable due to the closure.

Closures are commonly used in JavaScript for maintaining state, data privacy, and creating higher-order functions. They are a powerful feature of the language but can also lead to memory leaks if not used carefully, as the variable captured in closures are not garbage collected until the closure itself is destroyed.