When a new proposal is created, that proposal has to go through certain stages before it becomes part of the official specification. It’s important to keep in mind that in order for any proposal to move from one stage to another, a consensus among the TC39 must be met. This means that a large majority must agree while nobody strongly disagrees enough to veto a specific proposal.
Each new proposal starts off at Stage 0. This stage is called the Strawman stage. Stage 0 proposals are “proposals which are planned to be presented to the committee by a TC39 champion or, have been presented to the committee and not rejected definitively, but have not yet achieved any of the criteria to get into stage 1.” So the only requirement for becoming a Stage 0 proposal is that the document must be reviewed at a TC39 meeting. It’s important to note that using a Stage 0 feature in your codebase is fine, but even if it does continue on to become part of the official spec, it’ll almost certainly go through a few iterations before then.
The next stage in the maturity of a new proposal is Stage 1. In order to progress to Stage 1, an official “champion” who is part of TC39 must be identified and is responsible for the proposal. In addition, the proposal needs to describe the problem it solves, have illustrative examples of usage, a high level API, and identify any potential concerns and implementation challenges. By accepting a proposal for stage 1, the committee signals they’re willing to spend resources to look into the proposal in more depth.
The next stage is Stage 2. At this point, it’s more than likely that this feature will eventually become part of the official specification. In order to make it to stage 2, the proposal must, in formal language, have a description of the syntax and semantics of the new feature. In other words, a draft, or a first version of what will be in the official specification is written. This is the stage to really lock down all aspects of the feature. Future changes may still likely occur, but they should only be minor, incremental changes.
Next up is Stage 3. At this point the proposal is mostly finished and now it just needs feedback from implementors and users to progress further. In order to progress to Stage 3, the spec text should be finished and at least two spec complient implementations must be created.
The last stage is Stage 4. At this point, the proposal is ready to be included in the official specification. To get to Stage 4, tests have to be written, two spec complient implementations should pass those tests, members should have significant practical experience with the new feature, and the EcmaScript spec editor must sign off on the spec text. Basically once a proposal makes it to stage 4, it’s ready to stop being a proposal and make its way into the official specification. This brings up the last thing you need to know about this whole process and that is TC39s release schedule.
As of 2016, a new version of ECMAScript is released every year with whatever features are ready at that time. What that means is that any Stage 4 proposals that exist when a new release happens, will be included in the release for that year. Because of this yearly release cycle, new features should be much more incremental and easier to adopt.