順序なし、整数のみの計算式の文字列を実行します(evalの計算だけ版のようなもの)。実数はフリーズします

calc.js

const calc=((raw)=>{
  let s=[]
  const sw = (addIndex,v,f)=>new class {
      cases=new Map();
      def=null;
      c(v,f){
        this.cases.set(v,f)
        return this
      }
      d(f){
        this.def=f
        return this
      }
      eval(_f=()=>{}){
        _f(this,v)
        for (const [k,f] of this.cases.entries()){
          let r;
          if ((typeof k == "string" && v.startsWith(k)) || (k instanceof RegExp && (r=v.match(k)))){
            addIndex(k.length!=null ? k.length : r[0]?.length)
            return f(r)
          }
        }
        if (this.def)return this.def()
      }
    }().eval(f);
  for (let i=0;i<raw.length;){
    const t=sw((v)=>i+=v,raw.slice(i),(s)=>
      s.c(/^[0-9]+/,(v)=>parseInt(v[0])).c(/^[+\-*/]/,(v)=>v[0])
    )
    if (t)s.push(t)
  }
  let stack=[]
  for (let i=0;i<s.length;i++){
    if (typeof s[i] == "number")stack.push(s[i]);
    if (s[i]=="*")stack.push(stack.pop()*s[i+=1]);
    else if (s[i]=="/")stack.push(stack.pop()/s[i+=1]);
    else if (s[i]=="+")stack.push(stack.pop()+s[i+=1]);
    else if (s[i]=="-")stack.push(stack.pop()-s[i+=1]);
  }
  return stack.pop()
})