Uninitialized instance fields are not added to instances

[parzhitsky]

TypeScript Version: 3.9.0-dev.20200225

Search Terms: uninitialized not initialized class property in operator

Code

Looks like the same code that in JavaScript actually adds a non-static property to an instance, in TypeScript, however, does not.

class Thing {
	property;
}

const thing = new Thing();

console.log("property" in thing);

Expected behavior:

logs true

Actual behavior:

logs false

Playground Link: https://www.typescriptlang.org/play/?noImplicitAny=false&ts=3.9.0-dev.20200225#code/MYGwhgzhAEAqAWBLAdgc2gbwFAEgAOATgPZ4CmBALgJ4DcWAvllsEchBdBUmtALzTJSAdzjdUACgCUdZqwhEQpAHQgiEgESES5auugpOY6ViA

Related Issues: (none)

[parzhitsky]

Closed by accident :/

[IllusionMH]

same code that in JavaScript actually adds a non-static property to an instance

Class properties in TS and JS (with class fields proposal) differs (at least at this moment #34787)

If you want to have JS behavior you should set useDefineForClassFields to true.

[parzhitsky]

Seems like duplicate of #34787. Thanks @IllusionMH!