Connect Four JavaScript: Let the Drop Decide the Turn

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<body class="game"><main><header><div class="eyebrow">Original interactive state lab</div><h1>Connect Four JavaScript: Let the Drop Decide the Turn</h1><p>A full-column click should leave the same player on turn. A last-slot winning drop should win before the board is called a draw. <mark>Commit one legal drop before advancing play</mark> so the board and result agree.</p></header>
<nav><button type="button" id="drop0">Drop 1</button><button type="button" id="drop1">Drop 2</button><button type="button" id="drop2">Drop 3</button><button type="button" id="drop3">Drop 4</button><button type="button" id="drop4">Drop 5</button><button type="button" id="drop5">Drop 6</button><button type="button" id="drop6">Drop 7</button><button type="button" id="reset">Reset</button></nav><div id="board" class="board connect"></div><p id="result" class="result" role="status"></p><div id="ledger" class="ledger"></div>
<footer>Reloading starts an empty board with P1 to move.</footer></main><script>
(()=>{'use strict';const q=id=>document.querySelector('#'+id);const set=(id,value)=>{q(id).textContent=String(value);};const on=(id,callback)=>q(id).addEventListener('click',callback);const escapeHtml=value=>String(value).replace(/[&<>"']/g,c=>({'&':'&amp;','<':'&lt;','>':'&gt;','"':'&quot;',"'":'&#39;'}[c]));const table=(id,heads,rows)=>{q(id).innerHTML='<table><thead><tr>'+heads.map(h=>'<th scope="col">'+escapeHtml(h)+'</th>').join('')+'</tr></thead><tbody>'+rows.map(row=>'<tr>'+row.map(value=>'<td>'+escapeHtml(value)+'</td>').join('')+'</tr>').join('')+'</tbody></table>';};const field=(id,label,value='')=>'<label for="'+id+'">'+escapeHtml(label)+'</label><input id="'+id+'" value="'+escapeHtml(value)+'">';const btn=(id,label)=>'<button type="button" id="'+id+'">'+escapeHtml(label)+'</button>';const fmt=value=>Number(value).toFixed(6);
(function(){let board=[],turn=1,moves=0,terminal='';function render(message=''){q('board').innerHTML=[...board].reverse().flatMap(row=>row.map(v=>`<div class="cell p${v}">${v?'P'+v:'·'}</div>`)).join('');set('result',terminal||`P${turn} to drop; ${moves} moves. ${message}`);set('ledger','Bottom-up rows: '+board.map(row=>row.map(v=>v||'.').join(' ')).join(' / '));}function reset(){board=Array.from({length:6},()=>Array(7).fill(0));turn=1;moves=0;terminal='';render();}function drop(c){if(terminal)return;const r=board.findIndex(row=>row[c]===0);if(r<0)return render('Column '+(c+1)+' full; same turn retained.');board[r][c]=turn;moves++;for(const[dr,dc]of[[0,1],[1,0],[1,1],[1,-1]]){let n=1;for(const sign of[-1,1]){let rr=r+dr*sign,cc=c+dc*sign;while(rr>=0&&rr<6&&cc>=0&&cc<7&&board[rr][cc]===turn){n++;rr+=dr*sign;cc+=dc*sign;}}if(n>=4)terminal=`P${turn} wins on move ${moves}. Reset to play again.`;}if(!terminal&&moves===42)terminal='Draw: 42 legal drops without a winning line.';if(!terminal)turn=3-turn;render();}for(let i=0;i<7;i++)on('drop'+i,()=>drop(i));on('reset',reset);reset();})();})();
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