Remove | null from Typescript type

If you actually do something in the if block to make myCanvas non-null, TypeScript will recognize that:

let myCanvas = document.getElementById('point_fiel');
if (myCanvas == null) {
    return; // or throw, etc.
}
useHTMLElement(myCanvas); // OK

or

let myCanvas = document.getElementById('point_fiel');
if (myCanvas == null) {
    myCanvas = document.createElement('canvas');
}
useHTMLElement(myCanvas); // OK

Typescript typeguards also recognise the instanceof operator - useful when not-null isn't all you need to know

let myCanvas = document.getElementById('point_fiel');
if (myCanvas instanceof HTMLCanvasElement) {
  useHTMLElement(myCanvas);
} else if (myCanvas instanceof HTMLElement) {
  // was expecting a Canvas but got something else
  // take appropriate action 
} else {
  // no element found 
}