Cannot read property on Object (undefined?) inside instance

Question

When i try to access the object this.guy from the event function onMouveMove i get the following error:

"Uncaught TypeError: Cannot read property 'prop1' of undefined"

When i inspect with Chrome's Devtool, the value exist and is there, it just doesn't recognize inside the "onMouseMove" function.

MyClass = function(some_html_element){
  this.guy = {"prop1":[]};
  some_html_element.onmousemove = this.onMouseMove;
}
    
MyClass.prototype.onMouseMove = function(evt){
  if(!this.guy.prop1){    //<- here's the error
            alert("no error");
  }
}

a = new MyClass(document.querySelector("#example"))

<div id="example">
MOVE MOUSE ON ME
</div>

Any ideas ?

Tibrogargan · Accepted Answer · 2016-11-09 23:55:32Z

7

When you set the onMouseMove handler for some_html_element the function is not bound to your MyClass instance. You can fix this by explicitly binding it:

MyClass = function(some_html_element){
  this.guy = {"prop1":[]};
  some_html_element.onmousemove = this.onMouseMove.bind(this);
}

answered Nov 9, 2016 at 23:55

Tibrogargan

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3 Comments

Ruan Mendes Over a year ago

You should mark it as a duplicate, there are already plenty of questions exactly like this

Not a privileged user Over a year ago

wtf, i hate javascript now. Thanks!

Tibrogargan Over a year ago

@Notaprivilegeduser Don't hate. The way JavaScript binds (or doesn't bind) functions is actually really powerful once you get a handle on it.

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