Type guards in TypeScript are expressions that let you narrow down the type of a variable within a certain scope. This means you can tell the TypeScript compiler more about what type a value actually has at runtime, which is especially useful when working with union types or values that might be null or undefined. Type guards help you write safer code by preventing runtime errors that could occur if you try to use a value as the wrong type.

You need type guards because TypeScript's static type system cannot always know the exact type of a variable at runtime. For example, you might get a value from an API that could be a string or a number, or you might receive a parameter that could be an object with different shapes. If you try to use a method or property that does not exist on the actual runtime type, you will get a runtime error. Type guards allow you to check the type before using it, so you can avoid these mistakes.

TypeScript provides several built-in mechanisms for type narrowing, including:

• The typeof operator: checks if a value is a primitive type like "string", "number", or "boolean";
• The instanceof operator: checks if a value is an instance of a specific class or constructor;
• The Array.isArray() function: checks if a value is an array;
• The in operator: checks if a property exists in an object.

Besides these, TypeScript also narrows types based on truthiness. If you check if a value is truthy (for example, if (value) { ... }), TypeScript knows that inside the block, value is not null, undefined, false, 0, "", or NaN. Similarly, checking for null or undefined directly lets you narrow the type further, so you can safely access properties or call methods.

By using type guards, you make your code more robust and less likely to fail unexpectedly at runtime.