Is there a way to have TypeScript detect this issue? My strictNullChecks setting is enabled.
const foo: string = 'asdf';
if (foo !== null) {
console.log(foo);
}
If I modify that condition to:
if (foo !== 42) {
The error message displayed is:
This comparison will always evaluate to 'true' since the types 'string' and 'number' are not compatible.
The error-free nature of using === in TypeScript is due to specific allowances made for comparing values with null or undefined.
For situations where you want to make sure that null is a potential value, you can create a utility function using conditional types:
function checkNull<T>(value: T): null extends T ? boolean : void {
return (value === null) as any;
}
// Error
declare const example1: string;
if (checkNull(example1)) { }
// No Error
declare const example2: string | null;
if (checkNull(example2)) { }
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