TinyChan

UnicodeRange=(a,b)=>Array.from({length:b-a+1},(_,i)=>String.fromCharCode(a+i)).join("");
katakanaToHalfWidth=str=>{
	let s=0xFF65,t="・ヲァィゥェォャュョッーアイウエオカキクケコサシスセソタチツテトナニヌネノハヒフヘホマミムメモヤユヨラリルレロワン",
	    o="";
	loop:for(let i=0,c;i<str.length;i++){
		c=str[i],cc=c.charCodeAt(0);
		if(t.indexOf(c)>=0){
			o+=String.fromCharCode(s+t.indexOf(c));
		}else if(cc>=0x30AB&&cc<=0x30DD){ //dakutenﾞ and handakutenﾟ
			for(let j=1;j<3;j++){
				let f=t.indexOf(String.fromCharCode(cc-j));
				if(f>=0){
					o+=String.fromCharCode(s+f)+"ﾞﾟ"[j-1];
					continue loop;
				}
			}
			o+=c;
		}else if(cc>=0x21&&cc<=0x7E){ //basic Latin (full-width, feel free to comment out this)
			o+=String.fromCharCode(cc+(0xFF01-0x21));
		}else{
			o+=({"。":"｡","「":"｢","」":"｣","、":"､"})[c]??c;
		}
	}
	return o;
};
console.log((UnicodeRange(0x21,0x7E)+UnicodeRange(0x30A1,0x30FF)+"。「」、").split("").map(x=>x+" - "+katakanaToHalfWidth(x)).join("\n"));
console.log(katakanaToHalfWidth("チャスモハァーワ」。")); //ﾁｬｽﾓﾊｧｰﾜ｣｡