what is use for closure in javascript?

Question

Can any one tell me what is the use for closure and also tell me why to use this ?

Answer 1

In the JavaScript (or any ECMAScript) language, in particular, closures are useful in hiding the implementation of functionality while still revealing the interface.

For example, imagine you are writing a class of date utility methods and you want to allow users to lookup weekday names by index but you don't want them to see the array of names you use under the hood.

var dateUtil = {
  weekdayShort: (function() {
    var days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];
    return function(x) {
      if ((x != parseInt(x)) || (x < 1) || (x > 7)) {
        throw new Error("invalid weekday number");
      }
      return days[x - 1];
    };
  }())
};

Note that the days array could simply be stored as a property of the dateUtil object but then it would be visible to users of the script and they could even change it if they wanted, without even needing your source code. However, since it's enclosed by the anonymous function which returns the date lookup function it is only accessible by the lookup function so it is now tamper-proof.