How do I store changed values into a JavaScript object?

I have this JS object that is stored in a file called garage.js:

var myObject = {
 "cars": [
       {
        "type": "mustang",
        "body": {
          "length": 10.7,
          "width": 5.8,
          "color" : "#fff"
          }
        },
       {
        "type": "corvette",
        "body": {
          "length": 11.9,
          "width": 5.6,
          "color": "#000"
         }
   }]
};

When I make changes to the width wtihin a JS function within html, like this:

myObject.cars[0].body.width = 6

and then check the variable to see if the value has been updated, like this:

console.log(myObject.cars[0].body.width = 6)

the console prints 6... but it does not change the value in the stored object in garage.js... (how do I do this?)

The value appears to be represented locally but I need it to actually write this updated value back to the garage.js file so that it is stored permanently, so that when I refresh the html in the browser, the new value, 6, will have replaced the old value, 5.8, in the myObject object.

Do I need to use JSON.stringify and JSON.parse? ... or is it much simpler than that?

via Chebli Mohamed