Empty class properties are not initialized as undefined

[buschtoens]

TypeScript Version: 3.2.1

Search Terms: empty class property properties declared defined not initialized undefined

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

class Foo {
  property: string;
}

const foo = new Foo;

console.log('property' in Foo); // should be `true`

Expected behavior:

According to the class fields proposal, property should be initialized as undefined, as it does not have an initializer.

Considering that TypeScript wants to be a superset of JS, this is a problem and would be a breaking change.

I would really like to know what you think the golden path forward is here. When using @babel/plugin-transform-typescript with @babel/plugin-proposal-class-properties, the latter one has to be run first, otherwise the following code would behave incorrectly (taken from ember-cli-typescript):

class TestClass {
  // we shouldn't encourage folks to write code like this, but tsc ensures
  // that constructor param fields are set before field initializers run
  field = this.constructorParam;
  constructor(private constructorParam: string) {}
}

let instance = new TestClass('hello');
assert.equal(instance.field, 'hello');

But by running that transform first, the empty class property gets transformed to spec compliant this.property = undefined;.

This is a big problem, when dealing with derived classes that (re-)declare fields of base classes without defining them, as these would get transformed to defining them as undefined.

I know this is a somewhat unsupported setup, but TypeScript is diverging from the planned state of JS and I think this spec incompliancy, if intentional, should at least be explicitly documented and then "fixed" in Babel-land.

Actual behavior:

Class fields without initializers are stripped during transpilation.

Playground Link: Playground

Related Issues:

babel/babel#9105

[DanielRosenwasser]

Duplicate of #12437.

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.