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Is there anyway that I could check if a number already exists (and for instance, if there are more than one instance of a particular number, etc) in state with this code? I can´t figure out any, and I´ve tried all methods I could think of (Indexof, contains, etc, even turning value into an array didn´t help).

const button1 = [number: 'one', value: '1',number: 'two', value: '2',number: 'three', value: '3',number: 'divide', value: '/'];

const button2 = [number: 'four', value: '4',number: 'five', value: '5',number: 'six', value: '6',number: 'add', value: '+'];

const button3 = [number: 'seven', value: '7',number: 'eight', value: '8',number: 'nine', value: '9',number: 'subtract', value: '-'];

const button4 = [number: 'zero', value: '0',number: 'decimal', value: '.',number: 'equals', value: '=',number: 'multiply', value: '*'];


 class Calculator extends Component 
 constructor(props) 
 super(props);
 this.state = value: "0";
 this.handleClick = this.handleClick.bind(this);
 


 handleClick(evt) 
 const id = evt.target.id;
 const result = evt.target.value;

switch(id) 
 case 'clear':
 this.setState( value: "0");
 break;
 case 'equals':
 this.setState(prevState => (
 value: math.eval(this.state.value)
 ));
 break;

 default: this.setState(prevState => (
 value: `$prevState.value$result`.replace(/([/+-/*=])([/+-*=])/g, "$2")
 .replace(/^0+(?=[1-9])/, "")
 .replace(/^0+(?=.)/, "0")
 .replace(/^0+B/, "")
 .replace(/.+/g,".")
 )); 











render() 
 return(
 <div id="container">
 <Display value=this.state.value />
 <div>
 button1.map((el, index) => <Button onClick=this.handleClick key=el.index id=el.number value=el.value />)

 </div>
 <div>
 button2.map((el, index) => <Button onClick=this.handleClick key=el.index id=el.number value=el.value />)
 </div>
 <div>
 button3.map((el, index) => <Button onClick=this.handleClick key=el.index id=el.number value=el.value />)
 </div>
 <div>
button4.map((el, index) => <Button onClick=this.handleClick key=el.index id=el.number value=el.value />)
 </div>
 <div>
 <Button onClick=this.handleClick id="clear" value='clear' />
 </div>
 </div>
)

In Handle click, try replacing the default to below

 handleClick(evt) 
 const id = evt.target.id;
 let result = evt.target.value;

 switch (id) 
 case "clear":
 this.setState( value: "0" );
 break;
 case "equals":
 this.setState(prevState => (
 value: math.eval(this.state.value)
 ));
 break;

 default:
 this.setState(prevState => 
 const numbers = prevState.value.split(/[+-*=]/g);
 const lastNumber = numbers[numbers.length - 1];
 if (result === ".") ).length > 0) 
 result = "";
 
 
 let value = `$prevState.value$result`
 .replace(/([+-*=])([+-*=])/g, "$2")
 .replace(/^0+(?=[1-9])/, "")
 .replace(/^0+(?=.)/, "0")
 .replace(/^0+B/, "");

 return 
 value
 ;
 );
 
 

The method you're looking for is includes

EDIT per your additional information:

var positions = ;
for(i = 0; i < this.state.value.length; i++) 
 if(this.state.value.charAt(i) === result) 
 positions.push(i);
 

You'll have to edit to fit your specific needs (I get the impression you're looking for something specific which you haven't overtly stated) but I imagine this gets you past your obstacle.