How to detect that an array is replaced by an identical other array?

Question

In my code, I have a variable containing an array. I want to debug manipulations of that variable. I do not mind if the contents of the array are changing, but I have to see how often and when the array itself changes.

An example:

window.myarray = []
console.log(window.myarray); // outputs: []

window.myarray.push("bla"); // an irrelevant manipulation
console.log(window.myarray); // outputs: ["bla"]

window.myarray = ["bla"]; // a change of the array's identity!
console.log(window.myarray); // still outputs: ["bla"]

How can I detect, that the array that is stored in a variable has changed, even if the content of the old and new arrays are the same?

Store a temporary reference to window.myarray's state you want to detect it changing from. E.g tmp = window.myarray — mTv
– mTv, Commented Sep 10, 2018 at 11:05

mpm · Accepted Answer · 2018-09-10 11:13:44Z

5

You could use proxies

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

Or you can define a property on window

(function() {
  let _myArray;
  Object.defineProperty(window, "myArray", {
    set(value) {
      _myArray = value;
      console.log('changed', value);
    },
    get() {
      return _myArray
    }
  })
}())

window.myArray = [];

window.myArray = ['bar'];

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

edited Sep 10, 2018 at 11:13

answered Sep 10, 2018 at 11:10

mpm

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1 Comment

HMR Over a year ago

console.error would be slightly better as it includes the stack trace.

user5734311 · Accepted Answer · 2018-09-10 11:05:45Z

3

You check the identity like this:

window.myarray = []
var backup = window.myarray;

window.myarray.push("bla"); // an irrelevant manipulation
console.log(backup === window.myarray); // true

window.myarray = ["bla"]; // a change of the array's identity!
console.log(backup === window.myarray); // false

Note that backup isn't an actual copy, just a reference to the same in-memory object.

answered Sep 10, 2018 at 11:05

user5734311

4 Comments

slartidan Over a year ago

This is already helpful, but I would prefer to be able to compare it in the console log.

user5734311 Over a year ago

@slartidan I don't understand what you mean by that, at all.

slartidan Over a year ago

I would like to output something in the console log, that uniquely identifies an instance of an array. So that I can check afterwards if two logged arrays are identical or not.

user5734311 Over a year ago

@slartidan I see; according to this there's no way to grab the memory address like for instance Java prints as default toString().

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