The TypeScript team is only implementing features that have a chance of 100% of landing in JS or 0%. Therefore, they wait for Stage 3. If they start implementi

The TypeScript team is only implementing features that have a chance of 100% of landing in JS or 0%. Therefore, they wait for Stage 3. If they start implementing features at an earlier stage, there is the risk of implementing a feature with different semantics in TS than in JS, since the JS spec can still change (or event get rejected). Both cases will result in diverging languages, which is something they try to avoid. Edit: In the past, that didn't work that well. TS 3.8 is planned to ship with JS private fields support (the one with the # syntax). TS had private fields for a long time. In fact, it's one of the first things that the language had. However, these private fields behave entirety different to what ended up in the ES private fields spec. They can't change it afterwards. So in the future, we will have two semantically different ways of declaring a private field in a class in TS. Another case are decorators. They are still in stage 2 and may change. They already exist in TS, behind a flag. But every Angular application depends on their current implementation in TS. If the spec changes, it will get interesting.