People, when they come to TypeScript, might have different ideas in mind with how strict they want types checked.
Some people don’t want types at all, like JavaScript currently is, and want to use other aspects of TypeScript, while others might want to be very strict.
The default experience with TypeScript, where types are optional, inference takes the most lenient types, and there’s no checking for potentially null/undefined values. This is for the people who are looking for a more loose opt-in experience which can help validate only some parts of their program, and still have decent tooling.
TypeScript has several type-checking strictness flags that can be turned on or off, and all of our examples will be written with all of them enabled unless otherwise stated. The strict flag in the CLI, or "strict": true in a tsconfig.json toggles them all on simultaneously, but we can opt out of them individually. The two biggest ones you should know about are noImplicitAny and strictNullChecks.
noImplicitAny
Recall that in some places, TypeScript doesn’t try to infer types for us and instead falls back to the most lenient type: any. This isn’t the worst thing that can happen – after all, falling back to any is just the plain JavaScript experience anyway.
However, using any often defeats the purpose of using TypeScript in the first place. The more typed your program is, the more validation and tooling you’ll get, meaning you’ll run into fewer bugs as you code.
Turning on the noImplicitAny flag will issue an error on any variables whose type is implicitly inferred as any.
strictNullChecks
By default, values like null and undefined are assignable to any other type. While this can make writing some code easier, forgetting to handle null and undefined is the cause of countless bugs in the world.
The strictNullChecks flag makes handling null and undefined more explicit, and spares us from worrying about whether we forgot to handle null and undefined.
Strictness of Types was originally found on Access 2 Learn
