Outline
Let's walk through some TypeScript compiler options that I suggest that you make to your project to make things more strict in your codebase.
In the tsconfig.json file, find the CompilerOptions and add these additional options:
"noImplicitAny": true,
"noImplicitReturns": true,
"noImplicitThis": true,
"noFallthroughCasesInSwitch": true,
"strictNullChecks": true