Private-named instance fields

[mheiber]

@bloomberg teammates: The target for this PR is a branch that is currently up to date with Microsoft/TypeScript master.
Once this PR is approved, we can open a PR with the same changes from Bloomberg/private-named-instance-fields to Microsoft/private-named-instance-fields.

Private-Named Instance Fields

This PR implements part of the tc39 class fields proposal for TypeScript. It includes:

• parse and check private-named fields, methods, and accessors
• displayprivate names in the language server
• transform private-named instance fields

Example:

ts

class Greeter {
    #name: string;
    constructor(name: string) {
        this.#name = name;
    }
    greet() {
        console.log(`hello ${this.#name}`);
    }
}

const greeter = new Greeter("world");
console.log(greeter);                 // logs 'Greeter {}'
console.log(Object.keys(greeter));    // logs '[]'
greeter.greet();                      // logs 'hello world'

js

var _classPrivateFieldSet = function (receiver, privateMap, value) { if (!privateMap.has(receiver)) { throw new TypeError("attempted to set private field on non-instance"); } privateMap.set(receiver, value); return value; };
var _classPrivateFieldGet = function (receiver, privateMap) { if (!privateMap.has(receiver)) { throw new TypeError("attempted to get private field on non-instance"); } return privateMap.get(receiver); };
var _nameWeakMap_1;
"use strict";
class Greeter {
    constructor(name) {
        _nameWeakMap_1.set(this, void 0);
        _classPrivateFieldSet(this, _nameWeakMap_1, name);
    }
    greet() {
        console.log(`hello ${_classPrivateFieldGet(this, _nameWeakMap_1)}`);
    }
}
_nameWeakMap_1 = new WeakMap();
const greeter = new Greeter("world");
console.log(greeter); // logs 'Greeter {}'
console.log(Object.keys(greeter)); // logs '[]'
greeter.greet(); // logs 'hello world'

• We plan to open this PR against Microsoft/TypeScript as soon as this dependency is merged: Move class property transformation into new transformer file microsoft/TypeScript#30467
• Related issue
• Chrome 74 announcement (includes private class fields)
• tc39 class fields proposal

This PR includes work by the following engineers:

• Joey Watts @joeywatts (Bloomberg): private fields transformation
• Max Heiber @mheiber (Bloomberg): parser and checker, pr review
• Michael Gunter @Neuroboy23 (Bloomberg): pr review
• Rob Palmer @robpalme (Bloomberg): leadership, spec expertise
• Jaideep Bhoosreddy, Michael Molisani, Ravi Amin (Bloomberg)
• Ron Buckton @rbuckton (Microsoft): designed the checker implementation, pr review
• Daniel Rosenwasser @DanielRosenwasser (Microsoft): pr review

[Neuroboy23]

I see that this is the only explicit return value of undefined. Any other undefined would have to come from an inner method call. I see some existing calling code that does things like !getDeclarationName(...). Does this undefined value here mean the same thing as any other potential undefined values? If not, maybe it's better to throw here (explicitly or implicitly) instead?

[mheiber]

@Neuroboy23 these changes were reviewed internally months ago and also given a peruse by Daniel and Ron. I'm open to revisiting this after we open the PR against the MS TS repo.

[mheiber]

Re this specific question:

• a private name outside a class body is a parse error and will be caught by the parser. If the user has noEmitOnError: false in their compilerOptions then we have to do best-effort on the emit, even though the emit almost certainly bad. In this case, I'm not sure whether it would be better to return node or undefined.