TypeScript vs JavaScript: Why Type Safety Prevents Production Bugs

A client's JavaScript codebase had 14 production bugs in one month. Users could not check out. Profile updates failed silently. API responses crashed the frontend.

We converted the codebase to TypeScript. The compiler found 187 type errors before we ran the code. Fixing those errors eliminated 11 of the 14 bugs. The remaining three were logic errors TypeScript cannot catch.

TypeScript does not eliminate all bugs. It eliminates an entire category of bugs - the ones caused by passing the wrong type of data to functions. For JavaScript codebases of any meaningful size, that category accounts for 40-70% of production errors.

What TypeScript Actually Does

TypeScript is JavaScript with type annotations. You tell the compiler what type of data each variable, function parameter, and return value should be. The compiler checks your code before it runs.

JavaScript example:

TypeScript catches this:

The bug never reaches production because the code will not compile.

Categories of Bugs TypeScript Catches

Type Mismatch Bugs

JavaScript: You pass a string to a function expecting a number. JavaScript coerces types unpredictably. Sometimes it works (loose equality), sometimes it returns NaN, sometimes it throws runtime errors.

TypeScript: Compiler error before code runs.

Real example from client code:

TypeScript prevents this:

You are forced to convert types explicitly:

Null and Undefined Bugs