App.js

  let s = this.state.value;
  console.log("value of S",s);
  //for not allowing conscutive decimal in row
    if (e.target.value == "." && s.charAt(s.length - 1) == ".") {
      
      this.setState({
        value: s,
      });
    } //end of if
    
    //this is for making last oprator in calculation (test 13)
  else if( e.target.value == "+" ||
    e.target.value == "*" ||
    e.target.value == "/"){
  //5*-5=-25
  //5*-+5=10
  //5*-+5=5*+5=25
  // console.log(s)
  if(s.charAt(s.length-1)=="+"||s.charAt(s.length-1)=="-"||s.charAt(s.length-1)=="*"||s.charAt(s.length-1)=="/"){
    s=s.replace(s.charAt(s.length-1),"");
    if(s.charAt(s.length-1)=="+"||s.charAt(s.length-1)=="-"||s.charAt(s.length-1)=="*"||s.charAt(s.length-1)=="/"){
    s=s.replace(s.charAt(s.length-1),"");
  }
  }
  s+=e.target.value
console.log(s,e.target.value)
      this.setState({
        value:s
      })
      
    }

  //this for if user press ac then state value will be 0 so when user press any other number it should be replaced
    else {
      if(this.state.value=="0"){
        this.setState({
          value:e.target.value
        })
      }
      //if user press = and after that if he press any number it should start new calculation(taking the number pressed in the state value)
      else if((e.target.value=="0"||e.target.value=="1"||e.target.value=="2"||e.target.value=="3"||e.target.value=="4"||e.target.value=="5"||e.target.value=="6"||e.target.value=="7"||e.target.value=="8"||e.target.value=="9") && calculated==true){
      //calculated is boolean variable if = is presed the valye will be true
        calculated=false;
        // console.log("yes i am number pressed after calculation completed")
        // console.log("key i preseed",e.target.value);
        this.setState({
          value:e.target.value
        })
      
      }//if opreator is pressed after evaluation it should continue in the on going string
      //propblem is here whenver this execute s.charAt is not function shows
      else if( (e.target.value == "+" ||
      e.target.value == "-" ||
      e.target.value == "*" ||
      e.target.value == "/") && calculated==true){
      calculated==false;
        // console.log("you have pressed opreator")
        this.setState({
          value:this.state.value+e.target.value
        })
      }
      //if all above condition is wrong than it should add to the string
      else{

        this.setState({
        value: this.state.value + e.target.value,
      });
     
    }
    } //end of else