Logos Bear

You want a notificaion for every call to Array.push. The nice thing about JavaScript is that you can modify anything at anytime. So let’s ovrride the Array.push to give use our results.

Note: console.log uses Array.push, so if you put a console log inside of the method you will create an overflow.

var log = '';

Array.prototype.push = (function() {
  var native = Array.prototype.push;
  return function(item) {
    log = log + item;
    native.apply(this, arguments);
  }
})();

var a = [];
a.push(7);

log; // "7"

If you run this in the console and then use push to add another item to the array, you’ll notice that log has a lot of diffrent numbers in it. This is because console.log uses Array.push so you are getting interference.

There are two ways we can fix this. We can put the log as a property on the list, or we can use a custom version of the array that we use for logging.

log property

var a = [];

Array.prototype.push = (function() {
  var native = Array.prototype.push;
  return function(item) {
    this.log = this.log || '';
    this.log = this.log + item + ',';
    native.apply(this, arguments);
  }
})();

a.push(7);
a.log; // "7,"

Node: If your log looks something like "7,7,7,7,7,7,"then you still have the overridden version of Array. This is a prime example why overriding the native method is a bad idea.

Selective logging

Instead of modifing the native Array.push, let’s create a new array with logging.

var logArray = Object.create(Array.prototype);

logArray.push = function(item) {
  console.log('pushed ', item);
  a.log = a.log || '';
  a.log = a.log + '' + item;
  Array.prototype.push.apply(this, arguments);
};

var a = Object.create(logArray);
a.push(7); // "pushed  7"
a.log; // "7"

You might have noticed that we can use console.log here. This is because we are not overriding the native Array.push that console.log uses.

Note: If your log looks something like "undefined77,7,7,7,7,7," then you still have the overridden version of Array. This is a prime example why overriding the native method is a bad idea.