未捕获的类型错误：无法读取未定义的属性“hasOwnProperty”

Question

I'm using fabric.js to draw some text on a canvas. I have a function to create a text label. I'd like to have labels run a function when they're selected. The syntax for this is label.on('selected', functionToCall()); This works fine when I make the function an anonymous inner function, but when I break it out as a separate function, I get an Uncaught TypeError:

Cannot read property 'hasOwnProperty' of undefined.

What am I doing wrong?

Below is the code which doesn't work for me. Here's the broken code on jsfiddle, and a version set up with an anonymous function which works.

"use strict";

var canvas = new fabric.Canvas('c', {selection: false}),
  position = 50;

function onLabelSelection(theLabel) {
  if (theLabel.hasOwnProperty('edge')) {
    selectedEdge = theLabel.edge;
  } else {
    selectedEdge = null;
  }
}

function createLabel() {
  var label;
    
  label = new fabric.Text('Hello World', {
    left: position,
    top: position
  });
  position += 50;
    
  label.edge = null;

  label.on('selected', onLabelSelection(this));

  return label;
}

canvas.on('mouse:up', function() {
  var newLabel = createLabel(); 
  canvas.add(newLabel);
});

Answer 1

The syntax for this is label.on('selected', functionToCall())

No. The syntax for event handlers is to pass the handler function, not to call it:

label.on('selected', functionToCall);

You might want to try label.on('selected', onLabelSelection.bind(this)) or - since this inside createLablel is apparently undefined - just label.on('selected', onLabelSelection) .