Closed
Description
TypeScript Version: seen in 3.8.3, 3.9.2, 3.9.3, nightly
Search Terms: "jsdoc" "typescript" "strictnullcheck" "type guard"
Code
// @ts-check
// NOTE: Problem only happens when strictNullChecks is on.
// repro: https://www.typescriptlang.org/play?strictNullChecks=true&ts=3.9.3&useJavaScript=true#
/**
*
* @typedef Thing
* @property {number} id
*/
/**
* @param {Thing|null} variable
* @return {variable is Thing}
*/
const isNotNull = variable => variable !== null;
/**
* @param {Thing|null} thing
*/
const fn = thing => {
thing; // as expected, type is Thing | null
if (isNotNull(thing)) {
thing; // as expected, type is Thing
thing.id; // no error
}
const isAvailable = isNotNull(thing);
if (isAvailable) {
thing; // unexpected: type is Thing | null
thing.id; // error: Object is possibly null
}
};Expected behavior:
In the body of the second if, thing will be typed as Thing.
Actual behavior:
In the body of the second if, thing is typed as Thing | null
Playground Link: https://www.typescriptlang.org/play?strictNullChecks=true&ts=3.9.2&useJavaScript=true
Related Issues: no