How to suppress "error TS2533: Object is possibly 'null' or 'undefined'"?

You can suppress if needed, by adding a comment (with CAUTION below)

// @ts-ignore: Object is possibly 'null'.

Not a direct answer to the OP's question, but in my React application with Typescript - v3.6.2
tslint - v5.20.0

And using the following code

const refToElement = useRef(null);

if (refToElement && refToElement.current) {
     refToElement.current.focus(); // Object is possibly 'null' (for refToElement.current)
}

I moved on by suppressing the compiler for that line -

const refToElement = useRef(null);

if (refToElement && refToElement.current) {
     // @ts-ignore: Object is possibly 'null'.
     refToElement.current.focus(); 
}

CAUTION

Note that since it's a compiler error and not the linter error, // tslint:disable-next-line didn't work. Also, as per the documentation, this should be used rarely, only when necessary

UPDATE

With Typescript 3.7 onwards, you can use optional chaining, to solve the above problem as -

refToElement?.current?.focus();

Also, sometimes it just might be a matter of passing in the appropriate type to the generic paramenter, while using useRef.
Eg: In case of an input element -

const refToElement = useRef<HTMLInputElement>(null);

This feature is called "strict null checks", to turn it off ensure that the --strictNullChecks compiler flag is not set.

However, the existence of null has been described as The Billion Dollar Mistake, so it is exciting to see languages such as TypeScript introducing a fix. I'd strongly recommend keeping it turned on.

One way to fix this is to ensure that the values are never null or undefined, for example by initialising them up front:

interface SelectProtected {
    readonly wrapperElement: HTMLDivElement;
    readonly inputElement: HTMLInputElement;
}

const selectProtected: SelectProtected = {
    wrapperElement: document.createElement("div"),
    inputElement: document.createElement("input")
};

See Ryan Cavanaugh's answer for an alternative option, though!

If you know from external means that an expression is not null or undefined, you can use the non-null assertion operator ! to coerce away those types:

// Error, some.expr may be null or undefined
let x = some.expr.thing;
// OK
let y = some.expr!.thing;