@babel/typescript removed types which prevents decorator metadata from being emitted

Bug Report

Current Behavior It logs couldn't get the type :(.

Input Code

• Reproducible demo

import "reflect-metadata";

const logType = (target: any, key: string): void => {
  const type = Reflect.getMetadata("design:type", target, key);
  if (!type) {
    console.log("couldn't get the type :(");
    return;
  }
  console.log(`${key} type: ${type.name}`);
};

class Demo {
  @logType
  public someProperty: number = 0;
}

new Demo();

Expected behavior/code It should log someProperty type: Number.

Babel Configuration (.babelrc)

{
  "presets": ["@babel/env", "@babel/typescript"],
  "plugins": [
    ["@babel/plugin-proposal-decorators", { "decoratorsBeforeExport": true }],
    "@babel/proposal-class-properties",
    "@babel/proposal-object-rest-spread"
  ]
}

Environment

• Babel version(s): 7.2.0
• Node/npm version: doesn’t matter
• OS: doesn’t matter
• Monorepo: no
• How you are using Babel: babel-node/cli

Possible Solution There is a package babel-decorators-metadata which tries to solve this issue but at the same time breaks some stuff. There is also a question on stackoverflow regarding this issue.

Additional context/Screenshots @babel/typescript removes TypeScript type definitions before compilation process which causes packages such as reflect-metadata to not work correctly.

[nicolo-ribaudo]

Because there is an emitDecoratorMetadata set to true in the tsconfig.json

Oh ok. Babel doesn’t load the tsconfig.json file, but it transforms TS as if you were transforming it only with the isolatedModules: true option.

I suggest that you use something like this to define those metadata:

import "reflect-metadata";

const logType = (target: any, key: string): void => {
  const type = Reflect.getMetadata("design:type", target, key);
  if (!type) {
    console.log("couldn't get the type :(");
    return;
  }
  console.log(`${key} type: ${type.name}`);
};

class Demo {
  @logType
  @Reflect.metadata("design:type", Number)
  public someProperty: number = 0;
}

new Demo();

[leonardfactory]

Hi, I developed a plugin in order to emit decorators metadata, similar to TSC compiler, even if using babel with @babel/preset-typescript: babel-plugin-transform-typescript-metadata