call a javascript prototype object method from main class

Question

var log = function (e, clear) {
  if (clear === true) {
    this.clear();
  }
  console.log(e);
};
log.prototype = {
  clear: function () {
    console.clear();
  }
};

i'm calling like this.

log('hi', true);

it says clear is undefined. I also tried log.clear(); but same undefined.

Need to instantiate the object with `new` so the constructor gets called — Ryan, Oct 13 '14 at 02:41
`new Log.clear()` - Furthermore your log wrapper is weak. It should take a variable number of arguments, like printf in C — Ryan, Oct 13 '14 at 02:43
could be written as var log=console.log.bind(console) and be faster, more flexible, simpler... — dandavis, Oct 13 '14 at 02:54
More info on constructor functions and prototype here: http://stackoverflow.com/questions/16063394/prototypical-inheritance-writing-up/16063711#16063711 — HMR, Oct 13 '14 at 03:31

score 3 · Accepted Answer · answered Oct 13 '14 at 02:52

I'd write it like this

function Log(e, clear) {

  // make `new` optional
  if (!(this instanceof Log)) {
    return new Log(e, clear);
  }

  // clear?
  if (clear === true) {
    this.clear();
  }

  // log
  console.log(e);
}

Log.prototype.clear = function clear() {
  console.clear();
};

score 1 · Answer 2 · answered Oct 13 '14 at 02:44

1

When you invoke a function, without new operator, this will refer the default global object (in browsers window object), where clear function is not defined. That is why you are getting this error.

To make this refer an object created from the log constructor function, you need to call it like this.

new log("Error", true);

Now, JavaScript creates an Object and this will refer that newly created object. And when you do this.clear, it will first check if the current object has clear method, which it doesn't have. So, it will go up the prototype chain to find it in log's prototype and that function will be executed.

answered Oct 13 '14 at 02:44

thefourtheye

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previously i was not using prototype, and it was working fine. – Pritpal Singh Oct 13 '14 at 02:46
function log(e, clear) { clear = typeof clear === 'boolean' ? clear : false; if (clear === true) console.clear(); console.log(e); } – Pritpal Singh Oct 13 '14 at 02:51
@PritpalSingh You don't use `this` in this function :-) – thefourtheye Oct 13 '14 at 02:51

score 1 · Answer 3 · 2014-10-13T03:12:16.183

Simply putting a breakpoint on the this.clear(), or even just examining this in the console when the error terminated script execution, would have shown you that this was not set correctly.

The classic technique for defending against constructors not being called with new is

if (!(this instanceof log)) { return new log(e, clear); }

Minor point, but clear === true is semi-redundant. Boolean variables are themselves test conditions, without being compared to true or false.

call a javascript prototype object method from main class

3 Answers3