Handbook applyMixins code does not compile

[blimmer]

TypeScript Version: 3.8.3

Search Terms:

• Handbook Mixin
• Argument of type 'PropertyDescriptor | undefined' is not assignable to parameter of type 'PropertyDescriptor & ThisType'.

Expected behavior:
I expected the example in the Handbook to compile without problems.

Actual behavior:
Copying the suggested code from the docs/handbook failed to compile

Related Issues:

• I couldn't find any

Code

// See https://www.typescriptlang.org/docs/handbook/mixins.html
// Error is:
// Argument of type 'PropertyDescriptor | undefined' is not assignable to parameter of type 'PropertyDescriptor & ThisType<any>'.
//  Type 'undefined' is not assignable to type 'PropertyDescriptor & ThisType<any>'.
//    Type 'undefined' is not assignable to type 'PropertyDescriptor'.(2345)
function applyMixins(derivedCtor: any, baseCtors: any[]) {
  baseCtors.forEach((baseCtor) => {
      Object.getOwnPropertyNames(baseCtor.prototype).forEach((name) => {
      Object.defineProperty(derivedCtor.prototype, name, Object.getOwnPropertyDescriptor(baseCtor.prototype, name));
    });
  });
}

Output

"use strict";
// See https://www.typescriptlang.org/docs/handbook/mixins.html
// Error is:
// Argument of type 'PropertyDescriptor | undefined' is not assignable to parameter of type 'PropertyDescriptor & ThisType<any>'.
//  Type 'undefined' is not assignable to type 'PropertyDescriptor & ThisType<any>'.
//    Type 'undefined' is not assignable to type 'PropertyDescriptor'.(2345)
function applyMixins(derivedCtor, baseCtors) {
    baseCtors.forEach((baseCtor) => {
        Object.getOwnPropertyNames(baseCtor.prototype).forEach((name) => {
            Object.defineProperty(derivedCtor.prototype, name, Object.getOwnPropertyDescriptor(baseCtor.prototype, name));
        });
    });
}

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[blimmer]

It looks like we might just need a guard, like this:

function applyMixins(derivedCtor: any, baseCtors: any[]) {
  baseCtors.forEach((baseCtor) => {
    Object.getOwnPropertyNames(baseCtor.prototype).forEach((name) => {
      const baseCtorName = Object.getOwnPropertyDescriptor(baseCtor.prototype, name);
      if (!baseCtorName) {
        return;
      }
      Object.defineProperty(derivedCtor.prototype, name, baseCtorName);
    });
  });
}

[orta]

Fixed by my re-write of the mixins docs for the new site #719

[cx1111]

Hi @orta , it appears that #719 did not implement the typeguard from @blimmer above.

https://github.com/microsoft/TypeScript-Website/pull/719/files#diff-4f2ae2b0dc666e35310daac6f74a190dR193

The function from the latest version of the handbook still shows the same error as specified in the OP.