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Intermediate 16 min readModule: Module 5: Functions, Signatures & Overloads

Function Types, Callbacks & Overloads

Write type-safe functions with explicit return types, callback signatures, and overload definitions.

What You Will Learn in This Lesson

  • Typing higher-order callback arguments ((item: T) => boolean)
  • Declaring function overloads for multiple call signatures
  • Return type annotations vs automatic type inference

Introduction & Core Concept

Functions in TypeScript can specify parameter types, return types, and multiple overload signatures for polymorphic behavior.
WHY DOES THIS MATTER IN THE REAL WORLD?

Function overloads allow a single function to return different types based on the input argument type.

Function Overload Signatures

typescript
typescript
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function format(value: string): string;
function format(value: number): string;
function format(value: string | number): string {
  return typeof value === "number" ? `$${value.toFixed(2)}` : value.trim();
}
 
console.log(format(129.5));
console.log(format("  KWAS  "));

Line-by-Line Technical Breakdown

1The implementation signature at the bottom is not visible to callers.

Try It Yourself (Interactive Editor)

Modify the code in real-time and click Run to test live browser output and console logs.

Intelligent Code Runner & Live Sandbox[TYPESCRIPT]
TYPESCRIPT SOURCE EDITOR
Interactive Live Code

Industry Best Practices & Professional Standards

  • Always annotate return types on exported public API functions.

Lesson Summary & Core Takeaways

  • Function typing ensures type safety across functional invocation boundaries.