Remove .flowconfig file.

Remove flow from package.json:

  "scripts": {
-   "flow": "flow check",
-   "flowbuild": "flow-copy-source src dist",
-   "prebuild": "npm run docs && npm run clean && npm run flowbuild",
+   "prebuild": "npm run docs && npm run clean",
  },
  "devDependencies": {
-   "@babel/preset-flow": "^7.0.0",
-   "eslint-plugin-flowtype": "^2.50.0",
-   "eslint-plugin-flowtype-errors": "^3.5.1",
-   "flow-bin": "^0.81.0",
-   "flow-copy-source": "^2.0.2",
  }

Remove flow from .babelrc:

  "presets": [
-   "@babel/preset-flow"
  ]

Remove flow from .eslintrc:

  "extends": [
-   "plugin:flowtype/recommended",
-   "prettier/flowtype"
  ],
  "plugins": [
-   "flowtype",
-   "flowtype-errors"
  ],
  "rules": {
-   "flowtype-errors/show-errors": "error"
  }

Run yarn.

Remove documentation from package.json:

  "scripts": {
-   "docs": "documentation readme src --section=API",
-   "postdocs": "git add README.md",
-   "prebuild": "npm run docs && npm run clean",
+   "prebuild": "npm run clean",
  },
  "devDependencies": {
-   "documentation": "^8.0.0",
  }

Run yarn.

Install dependencies:

yarn add -D @babel/preset-typescript @types/jest @typescript-eslint/eslint-plugin @typescript-eslint/parser typescript

Update package.json:

+ "types": "dist/ts/src",
  "scripts": {
+   "type-check": "tsc --noEmit",
-   "lint": "eslint .",
+   "lint": "eslint . --ext js,ts,tsx",
-   "build": "babel src -d dist",
+   "build": "tsc --emitDeclarationOnly && babel src -d dist -x .js,.ts,.tsx",
  },
  "lint-staged": {
-   "*.js": [
+   "*.{js,ts,tsx}": [
-     "eslint --fix",
+     "eslint --fix --ext js,ts,tsx",
      "git add"
    ]
  }

Create tsconfig.json

{
  "compilerOptions": {
    "outDir": "dist/ts",
    "target": "esnext",
    "module": "esnext",
    "moduleResolution": "node",
    "jsx": "react",
    "strict": true,
    "declaration": true,
    "noFallthroughCasesInSwitch": true,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "stripInternal": true
  }
}

Update .babelrc:

  "presets": [
+   "@babel/preset-typescript"
  ]

Update .eslintrc with these settings:

  "settings": {
    "import/resolver": {
      "node": {
        "extensions": [".js", ".jsx", ".ts", ".tsx"]
      }
    }
  },
  "overrides": [
    {
      "files": ["**/*.ts", "**/*.tsx"],
      "parser": "@typescript-eslint/parser",
      "parserOptions": {
        "project": "./tsconfig.json"
      },
      "plugins": [
        "@typescript-eslint"
      ],
      "rules": {
        "no-undef": "off",
        "no-unused-vars": "off",
        "no-restricted-globals": "off"
      }
    }
  ]