Step 5 - Null, undefined and boolean operators

Let’s proceed with step 5

(5) Add safety to boolean expressions, and improve handling of null and undefined

• To catch problematic truthy/falsy expressions, enable the ESLint rule @typescript-eslint/strict-boolean-expressions
• Enable the tsconfig option strictNullChecks to ensure that if null is desired to be an allowed value in a type, it has to be explicitly stated as such
• Enable the tsconfig option exactOptionalPropertyTypes to catch occurrences where optional properties are explicitly set to the value undefined instead of being deleted

There are a few places where you’ll need to change if (!foo) to if (typeof foo === 'undefined'). Use care when deciding whether null should be handled specifically — it’s easy to make mistakes!

Fix compiler and lint errors until all of the important commands succeed

sh
yarn typecheck
yarn lint
yarn test

This is the last of the “tightening up strictness” steps we’ll complete together, because they all pretty much feel this way, and we have more interesting things to discuss.