Quick Answer

TypeScript detects unreachable code path

Understanding the Issue

TypeScript's type checker can detect conditions that will always evaluate to false based on the types involved, helping catch logical errors.

The Problem

This code demonstrates the issue:

Typescript Error 
// Problem: Impossible condition
function process(x: string) {
    if (x === null) { // Warning
        console.log("null");
    }
}

The Solution

Here's the corrected code:

Typescript Fixed 
// Solution 1: Correct the type
function process(x: string | null) {
    if (x === null) { // Valid
        console.log("null");
    }
}

// Solution 2: Remove impossible check
function process(x: string) {
    console.log(x.toUpperCase());
}

Key Takeaways

Review logic when TypeScript flags impossible conditions.