How to prevent a member from being undefined within a constructor using TypeScript?

Question

I have introduced a bug by writing a class like this:

class SomeClass {
    private readonly item: string;

    constructor(item: string) {
        // bug, item is never  assigned
    }

    public getInfo(): string {
        return this.item; // always returns `undefined`
    }
}

the item is never assigned and hence each call to getInfo() would return undefined. This code transpiles successfully though.

The code style of my current project is preventing the usage of the short-hand constructor via tslint's no-parameter-properties rule, and hence I cannot do:

class SomeClass {
    public constructor(private readonly item: string) {
    }

    public getInfo() {
        return this.item;
    }
}

I was expecting tsc to throw an error due to the strictNullChecks setting of my tsconfig.

Is there a way to make typescript detect this bug and mark its compilation as an error?

This is my current tsconfig.json compilerOptions:

  "compilerOptions": {
    "target": "ES6",
    "lib": [
      "es6",
      "es2017",
      "DOM"
    ],
    "module": "commonjs",
    "pretty": true,
    "outDir": "dist",
    "sourceMap": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "strictNullChecks": true,
    "forceConsistentCasingInFileNames": true,
  }

Answer 1

If you have tsc >=2.7.1, then you are either looking for the compiler options

--strict

Enable all strict type checking options. Enabling --strict enables --noImplicitAny, --noImplicitThis, --alwaysStrict, --strictNullChecks, --strictFunctionTypes and --strictPropertyInitialization.

as that contains all the strict rulesets.

Or more specifically for

--strictPropertyInitialization

as that one is designed for your use-case:

Ensure non-undefined class properties are initialized in the constructor. This option requires --strictNullChecks be enabled in order to take effect.

With that setting, tsc will now throw:

src/SomeClass.ts:2:22 - error TS2564: Property 'item' has no initializer and is not definitely assigned in the constructor.

2     private readonly item: string;
                       ~~~~