Mastering TypeScript 3 - Third Edition

noImplicitReturns

If a function has declared that it will return a value, then the compiler can ensure that it does, indeed, return a value by using the noImplicitReturns option. Often, when we are working with a large function, or if we are re-factoring an existing function, we may inadvertently introduce unwanted logic flaws if we are not careful. Consider the following code:

function isLargeNumber(value : number) : boolean | undefined { 
    if (value > 1_000_000) 
        return true; 
} 

Here, we have defined a function named isLargeNumber that can be invoked with a single parameter named value. This function can return either a boolean or undefined. Note that in general, this function should only return a boolean, but for the sake of this discussion, ...