When it comes to TypeScript, there is a limitation in its intelligence when it comes to proving that the variable x in the second if statement is definitely a string. The function prints only accepts a string, and since TypeScript cannot conclusively determine that x is a string, an error is thrown.
In cases where type checking occurs within the condition of an if statement, TypeScript handles it well due to a specific mechanism designed for this scenario. However, inferring the type of one variable based on certain values of another variable proves to be too complex for TypeScript's capabilities. While it may seem straightforward in some instances, it becomes progressively challenging and potentially unfeasible in broader contexts.
If you insist on making the second case viable even with separate type checks from the if statement, providing additional information to TypeScript through explicit casting becomes necessary. For example, using prints(x as string) communicates to TypeScript, "I assure you that this is always a string. If not, any failure is attributed to me." This type of cast signals to TypeScript that the developer possesses insights beyond its comprehension and places trust in those decisions.