Promotion callback

[barrust]

It would be helpful if one could pass a either a value ('q') or a function to the promotion option in move. This would allow for generating the answer to what to promote to when necessary instead of having to default it to something.

[jhlywa]

You must pass the promotion piece when promoting; chess.js doesn't default. You can use one of the following:

var chess = new Chess();

chess.load('8/1P5k/8/8/8/8/8/1K6 w - - 0 1')
chess.move('b8=N')
// -> { color: 'w',  from: 'b7',  to: 'b8',  flags: 'np',  piece: 'p', promotion: 'n',  san: 'b8=N' }

chess.load('8/1P5k/8/8/8/8/8/1K6 w - - 0 1')
chess.move({from: 'b7', to: 'b8', promotion: 'r'});                   
// -> { color: 'w',  from: 'b7',  to: 'b8',  flags: 'np',  piece: 'p', promotion: 'r',  san: 'b8=R' }

chess.load('8/1P5k/8/8/8/8/8/1K6 w - - 0 1');                                                               
chess.move('b7b8=q', {sloppy: true});   
// -> { color: 'w',  from: 'b7',  to: 'b8',  flags: 'np',  piece: 'p', promotion: 'q',  san: 'b8=Q' }          

Does this suffice?

[barrust]

Sorry, I should have been more clear about the term default. I have been using the second version described while making a UI to help teach my kids about chess. What I was thinking would be a great additon, would be something like this:

var chess = new Chess();
chess.load('8/1P5k/8/8/8/8/8/1K6 w - - 0 1')
chess.move({
    from: 'b7', 
    to: 'b8', 
    promotion: function() {
        var promotion_options = ['q', 'r', 'n', 'b'];
        var promotion_val = promotion_options[Math.ceil(Math.random()*promotion_options.length) - 1];
        return promotion_val;  // this is a random promotion but it could also have been more clever...
    }
}); 

Does that make sense? It would allow for the developers of a UI to give the user a choice of promotion pieces after the promotion is signaled.

[barrust]

The random char function is just to show what I meant by not knowing the value before the move is registered. chess.js does not need to support the callback functionality and I am fine if it is decided that it is not within scope.

Thanks for your help and a great library

[kwende]

Yeah, I'm sorry, but @barrust is really right on this. You ought to be able to capture the fact someone is about to promote a piece and choose what piece to promote via a callback. "Why does Chess.js need to run a callback for this?" doesn't understand how people are using the library.