Reflect.getMetadata is not a function

I ran into this problem inside a lib. I resolved it by following @bashleigh’s example and installing the types,

npm i --save @types/reflect-metadata

But also had to add an entry within tsconfig.lib.json:

{
  "extends": "../../tsconfig.json",
  "compilerOptions": {
    ...
    "types": [
      "reflect-metadata"
    ]
  }
}

I added the reflect-metadata types and that resolved my problem yarn add --dev @types/reflect-metadata