TypeScript uses truthiness checks and explicit null or undefined checks to narrow types within your code. When you use a value in a condition—such as an if statement—TypeScript analyzes the possible types and removes null and undefined if the value is checked for truthiness. This means that inside the block where a value is confirmed to be truthy, TypeScript treats it as a non-null, non-undefined type, allowing you to safely access its properties or methods. This narrowing is essential for writing safe code that avoids runtime errors, especially when dealing with values that may be optional or missing. By leveraging these checks, you can prevent issues like trying to read properties from null or undefined, which would otherwise cause your program to crash.