Why can't I call this function in React? [duplicate]

I'm getting the error:'TypeError: this.createCanvas is not a function'.

I don't understand why. I've bound it in the constructor.

import React, { Component } from 'react';

class ImageEditor extends Component {

constructor() {
    super();
    this.createCanvas = this.createCanvas.bind(this);
    this.getImageDimensions = this.getImageDimensions.bind(this);
}

componentDidMount() {
    const imageurl = "https://storage.googleapis.com/" + this.props.location.state.imageuri;
    this.getImageDimensions(imageurl);
}

getImageDimensions(url) {
    var img = new Image();

    img.onload = function() {
        this.createCanvas(this.width, this.height);
    }

    img.src = url;
}

createCanvas(width, height) {
    let mycanvas = document.createElement("canvas");
    mycanvas.width = width;
    mycanvas.height = height;
    mycanvas.id = "mycanvas";
    document.body.appendChild(mycanvas);
}

render() {
    return (
        <h1>{this.imageurl}</h1>
    )
}

}

export default ImageEditor;