How to late init non-nullable variables?

Question

In my tests, I often have to declare a variable at the top, which I will use in the beforeAll and the afterAll.
Example:

describe('My test suite', () => {
  let app: Server;
  beforeAll(async () => {
    app = await Server.init();
  });
  afterAll(async () => {
    await app.close();
  });
});

The problem is that when I turn on strictNullChecks, I get a warning on the line let app: Server; "Assigning nullable value to non-nullable variable".
Is there an equivalent to the late keyword in dart?

Of course, I can do let app: Server | undefined; but then I have to app!.close(). I'm wondering if there is a better way to do this.

Edit
Here is the base tsconfig.json

// tsconfig.json
{
  "compilerOptions": {
    "module": "commonjs",
    "allowSyntheticDefaultImports": true,
    "declaration": true,
    "removeComments": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "target": "es2017",
    "sourceMap": true,
    "outDir": "./dist",
    "baseUrl": "./",
    "incremental": true,
    "esModuleInterop": true,
  },
  "exclude": [
    "node_modules",
    "dist",
  ]
}

I extended it to use it with ESLint:

// tsconfig.eslint.json
{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "strictNullChecks": true
  }
}

And then my eslintrc.js:

module.exports = {
  env: {
    es6: true,
    node: true,
  },
  extends: ['airbnb-base'],
  ignorePatterns: ['.eslintrc.js'],
  globals: {
    Atomics: 'readonly',
    SharedArrayBuffer: 'readonly',
  },
  parser: '@typescript-eslint/parser',
  parserOptions: {
    ecmaVersion: 2018,
    sourceType: 'module',
    project: './tsconfig.eslint.json',
  },
  plugins: [
    '@typescript-eslint',
    'strict-null-checks',
  ],
  rules: {
    'strict-null-checks/all': 'warn',
    // Turn off no-shadow because of enum
    // See: https://github.com/typescript-eslint/typescript-eslint/issues/325
    'no-shadow': 'off',
    'no-dupe-class-members': 'off',
    '@typescript-eslint/no-dupe-class-members': ['error'],
    '@typescript-eslint/explicit-function-return-type': ['warn', {
      allowExpressions: true,
    }]
  },
  settings: {
    'import/resolver': {
      node: {
        extensions: ['.js', '.jsx', '.ts', '.tsx'],
      },
    },
  },
};

Answer 1

You can use the bang non-null assertion character before the : used to declare the type of a variable to tell the TypeScript system that you are handling the nullability:

describe('My test suite', () => {
  let app!: Server;
  beforeAll(async () => {
    app = await Server.init();
  });
  afterAll(async () => {
    await app.close();
  });
});