The Temporal Dead Zone, or why the TypeScript codebase is full of var statements
3 days ago
- #JavaScript
- #Performance
- #TypeScript
- The Temporal Dead Zone (TDZ) is a concept in JavaScript where variables declared with `let` and `const` are not accessible before their initialization.
- Variables declared with `var` do not have a TDZ, leading to potential issues like variable leakage and undefined values when accessed before initialization.
- TypeScript's codebase uses `var` statements extensively for performance reasons, as avoiding TDZ checks can lead to significant performance improvements (e.g., 8% in some benchmarks).
- Using `var` in modern JavaScript is generally discouraged due to its lack of block scoping and immutability enforcement, favoring `let` and `const` instead.
- The TDZ is a beneficial feature that prevents accessing uninitialized variables, but it comes with a runtime performance cost.