How to understand closures in Javascript? [duplicate]

问题

How can one understand closures in Javascript?

In general terms, a closure is a function bound to one or more external variables. When it is called, the function is able to access these variables. In JavaScript, closures are often implemented when functions are declared inside another function. The inner function accesses variables of the parent one, even after the parent function has terminated

In this statement, "a closure is a function bound to one or more external variables", does it mean we can do this : var myFun = Function(msg){...}; is it correct ?

What does it mean "even after the parent function has terminated"?

回答1:

closure is a function bound to one or more external variables

An example of this concept is that the function bar is bound to the external variables x, y, and z:

function foo(x, y) {
  var z = 3;

  return function bar(a, b, c) {
    return (a + b + c) * (x + y + z);
  };
}

var closure = foo(1, 2);
closure(5, 6, 7); // (5 + 6 + 7) * (1 + 2 + 3) = 24

The variable closure refers to the inner function bar returned from the call to foo. Invoking closure is like reentering the scope within foo, which gives visibility into all of foo's local variables and parameters.

even after the parent function has terminated

This means that after foo is executed, the returned function stored in the closure variable persists the state of foo. You can even create multiple independent closures by invoking foo again:

var closure = foo(1, 2);
closure(5, 6, 7); // (5 + 6 + 7) * (1 + 2 + 3) = 24

var closure2 = foo(0, 0);
closure2(5, 6, 7); // (5 + 6 + 7) * (0 + 0 + 3) = 21

/* closure2 does not affect the other closure */
closure(5, 6, 7); // (5 + 6 + 7) * (1 + 2 + 3) = 24

回答2:

I'm not sure where you are quoting from, but it sounds like it's referencing when the parent function has finished running.

回答3:

Your interpretation of external variables is incorrect. It really means it can do this:

function make_closure() {
  var x = 20;
  return function() {
    console.log(x);
  };
}

var closure = make_closure();
closure(); // Displays 20

来源：https://stackoverflow.com/questions/15667093/how-to-understand-closures-in-javascript

标签

javascript

closures