it would look nice

It can be solved, just replace draw in function animate:

if (to_row * this.circleRadius >= cur_pos) {
            this.clear();
            //this.draw(); current call position
            this.drawCircle(this.circleRadius * column + 50, cur_pos + 50, 25, fg_color, "black");
            this.drawMask();
            this.draw(); // call draw() there to solve radius changing
            window.requestAnimationFrame(function () {
                that.animate(column, move, to_row, cur_pos + 25, callback);
            });
        } else {
            callback();
        }

Not sure if it counts as a bug, but I find it too obvious.
Just placed that red on the 6th column and the game put a blue on the first. Or was it by design?

Log, if can help: http://pastebin.com/AAAjUXEw

When player 1 puts a chess in the middle column, and then it is always placed on the top of player 2, it is always a draw. I don't know if this is a bug

At README.md, "Browser compatibility" section, it still mentions

Require browser that supports ES2015's Promise.

This line at README.md should be removed because I have already included a Promise polyfill in https://github.com/kenrick95/c4/blob/master/src/app.ts#L1

When column is full, player can still choose the column and the game hangs (game state does not change, no player can do action).

• Add checking before accepting the action to fill the full column.

Image & title describes everything

Allow users to input a username when they start a game.

I would like to translate this project, even though it is simple enough. I want to share this project with my friends who are not very good at English, how do I go about doing that? Thanks!
One more question, can this project be done through web server only, using yarn to run the project is not as easy to manage on the server as on the web. I really like this project and would like to get a reply.

Motivation

On the README section, after installing dependencies, it is said to generate dist/ folder and open index.html in the browser.

I saw that #7 moved the project to Parcel. Hence, simple yarn start should suffice and it will even enable live reload. Available at http://localhost:1234/

Hey,

I am new to TypeScript and open source and I found your game to be an amazing repository to learn and practice TypeScript by refactoring and writing tests for your code.

I would like to contribute a little to the repo on a daily basis and would love it if you could review my code daily and suggest changes.

It would be nice if one could play against another human over the network / Internet or however they have access to the server running the game. If you plan for something like this on your own, I encourage you to implement this.
If you do not plan this on your own yet, I am willing to make this my own task and submit a PR when I am ready.
Please let me know what you think about it. :)