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); // OKor
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 }