Here’s all of the chess engine code, it’s a modified version of the 1.25K game on Oscar Toledo G.’s site, where you’ll find all his award-winning tiny chess programs and an ebook explaining how they work.

for(B=y=u=b=0,x=10,z=15,I=[],l=[];l[B]=("ustvrtsuqqqqqqqq"+"yyyyyyyy}{|~z|{}@G@TSb~?A6J57IKJT576,+-48HLSUmgukgg OJNMLK IDHGFE").charCodeAt(B)-64,B++<120;I[B-1]=B%x?B/x%x<2|B%x<2?7:B/x&4?0:l[u++]:7);X=(c,h,e,S,s)=>{c^=8;for(var T,o,L,E,D,O=20,G,N=-1e8,n,g,d=S&&X(c,0)>1e4,C,R,A,K=78-h<<9,a=c?x:-x;++O<99;)if((o=I[T=O])&&(G=o&z^c)<7){A=G--&2?8:4;C=9-o&z?l[61+G]:49;do{R=I[T+=l[C]];g=D=G|T+a-e?0:e;if(!R&&(G||A<3||g)||(1+R&z^c)>9&&G|A>2){if(!(2-R&7))return K;for(E=n=G|I[T-a]-7?o&z:6^c;E;E=!E&&!d&&!(g=T,D=T<O?g-3:g+2,I[D]<z|I[D+O-T]|I[T+=T-O])){L=(R&&l[R&7|32]*2-h-G)+(G?0:n-o&z?110:(D&&14)+(A<2)+1);if(S>h||1<S&S==h&&L>2|d){I[T]=n,I[g]=I[D],I[O]=D?I[D]=0:0;L-=X(c,h+1,E=G|A>1?0:T,S,L-N);if(!(h||S-1|B-O|T-b|L<-1e4))return W(I,B=b,c,y=E);E=1-G|A<7|D|!S|R|o<z||X(c,0)>1e4;I[O]=o;I[T]=R;I[D]=I[g];D?I[g]=G?0:9^c:0}if(L>N||!h&L==N&&Math.random()<.5)if(N=L,S>1)if(h?s-L<0:(B=O,b=T,0))return N}}}while(!R&G>2||(T=O,G|A>2|z<o&!R&&++C*--A))}return-K+768<N|d&&N};Y=(V)=>{X(8,0,y,V);X(8,0,y,1)};Z=(U)=>{b=U;I[b]&8?W(I,B=b):X(0,0,y,1)}