typescript-examples.md

Testing TypeScript examples

Challenges:

• Testing expected static errors. In JavaScript, expected dynamic errors can be tested via:
  • assert.throws(() => eval('const myVar;'), SyntaxError);
  • assert.throws(() => null.someProp, TypeError);
• Testing expected inferred types.

Approach:

• Pass 1—runtime errors: Run the examples as unit tests; for example, via ts-node and Mocha or AVA.
• Pass 2—static errors: Use a static checker for the unit test code.
  • I’m working on such a checker and will hopefully have time to open-source it.

Static checker: expected static errors

Approach: Ingore the errors via @ts-ignore

function func1(x: Object) { }
func1('abc'); // OK

function func2(x: object) { }
//@ts-ignore: Argument of type '"abc"' is not assignable to parameter of type 'object'. (2345)
func2('abc');

The static checker warns if the static error doesn’t match text and number after @ts-ignore:.

(This functionality would be useful for TypeScript code in general.)

Static checker: expected inferred types

//%inferred-type: Object
const obj1 = new Object();

//%inferred-type: object
const obj2 = Reflect.getPrototypeOf({});

//%inferred-type: {}
const obj3 = {};

//%inferred-type: () => number
function myFunc() {
  return 123;
}

The static checker warns if the inferred type doesn’t match what’s mentioned after %inferred-type:.

Related tools

These tools are related:

• TypeScript TwoSlasher: https://github.com/microsoft/TypeScript-Website/tree/v2/packages/ts-twoslasher
  • Example: https://www.typescriptlang.org/v2/en/tsconfig#strictFunctionTypes
  • Upsides:
    • Powerful
    • Awesome for web content (tooltips, colors, etc.)
  • Downsides:
    • Doesn’t tie errors to specific lines in the code
    • Less suited for PDFs and print
• dtslint: https://github.com/microsoft/dtslint
  • Upside: standard
  • Downside:
    • No way to check for specific errors
    • Slightly more work to set up (AFAICT)
• tsd: https://github.com/SamVerschueren/tsd
  • Downside:
    • No way to check for specific errors
    • Unsure if its custom TypeScript version changes how other code behaves

Anything missing?

So far, this approach covers all of my needs for the following TypeScript blog posts:

• https://2ality.com/2020/01/typescript-enums.html
• https://2ality.com/2020/01/special-values-typescript.html
• https://2ality.com/2020/01/typing-objects-typescript.html

Is anything missing?