Question

I’m new to JavaScript, and am having a problem understanding this code:

function addProperty(o) {
   var value;

   o["get"] = function()  { return value; }
   o["set"] = function(v) { value = v; }
}

var a = {};
addProperty(a);
var b = {};
addProperty(b);

a.set(4);
b.set(5);
print("a is " + a.get() + "; b is " + b.get());

This prints (in v8/d8) a is 4; b is 5. If I comment out the var value; line, I get a is 5; b is 5. Where is the ‘value’ object, and why are there two of them? Thanks.

Answer 1

The value variable is local to addProperty. The first time addProperty is called, a new value is created, over which both of the functions close. The second time addProperty is called, a second value is created over which two new functions close.

Removing var creates a global value on the window object that is shared by all of the functions.

Maybe you mean to do this:

function createPropertyMgr() {
    var value;
    return function(o) {
       o["get"] = function()  { return value; }
       o["set"] = function(v) { value = v; }
    }
}

var addProperty = createPropertyMgr();

This new addProperty function closes over one value no matter how many times it’s called. I’m not sure I understand the use case, but that should demonstrate the difference.