TC39 is the 39th technical committee of an organization called ECMA International. ECMA(European Computer Manufacturers Association) is a standardizing body like ISO. TC39 is the committee that designs ECMAScript.
The TC39 committee is made up of people who are sent by member organizations that are companies and non-profits who are part of ECMA. The companies include browser manufacturers also. So the TC39 committee members either work for or are associated with these companies that are part of ECMA International.
A new feature or a proposal to ECMAScript can be suggested by anyone. The proposal then goes through 5 stages before standardization, starting with stage 0. To move the proposal from one stage to next stage, approval from TC39 is required.
Stage 0 is also know as strawperson stage. This is the start for submitting a proposal. A proposal is submitted in a free-form way. The submission can be done either by a TC39 member or a non-member through this discourse group or connecting through Matrix chat room.
TC39 Committee owns proposals at stage 1 and beyond. At stage 1, a champion is identified for a proposal who takes care of the advancement of the proposal. During this stage following things happen:
We can view the proposals that are in stage 1 here. In the list of proposals, you can also see the name of author and champion.
By the time a proposal reaches Stage 2, the initial specification for the proposal should be ready. During this stage, the TC39 precisely describe the syntax and semantics using the formal specification.
So, if a proposal reaches Stage 2, it means that the committee is expecting this proposal to be included in the standard eventually.
You can view the list of proposals that are in Stage 2 in TC39 Proposals repository.
If a proposal enters Stage 3 means,
We start seeing this specification implemented in some browsers. The feedback from implementation is then used to further refine the proposal.
Whenever a new feature is added to the standard, corresponding test scripts are added to Test262. For a stage 4 proposal, the test scripts should be complete. If you are interested, you can see the report of running the test suites online.
There should be atleast two compatible implementations which pass the acceptance test. Also, a pull request is sent to ecma262 repo with updated spec text. All ECMAScript editors needs to approve this pull request.
Once a proposal reaches Stage 4, it will be included in the specification. ECMAScript General Assembly approves the new standard in July every year.