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First click on item console log item inside arr. Second click returns error uncaught type error includes is not a function. And console log arr is 1.

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constructor(props){
super(props)
this.state = {
  arr = []
this.handleClick = this.handleClick.bind(this);
handleClick(e) {
  let arr = this.state.arr
  let item = e.target.id
  if (!arr.includes(item)) {
    this.setState({arr: arr.push(item)})
  } else {
    this.setState({arr: arr.filter(x => x !== item)})
                item.push(item) ?? edit also just noticed that the object literal that initializes this.state is also a syntax error.
– Pointy
                Oct 10, 2018 at 15:45
                You keep changing the code.... is it what you actually have, or what you are trying!!!!!!!
– epascarello
                Oct 10, 2018 at 16:05
                this.setState({arr: arr.push(item)}) is WRONG because you are setting what push  returns.....
– epascarello
                Oct 10, 2018 at 16:12
react says that data is inmutable, also your handle click by far as I see is not binded, thus do this 
handleClick = (e)=> {
  let arr = this.state.arr
then make a copy of state
handleClick = (e)=> {
  let arr = Object.assign([],this.state.arr)
and at the end set state of data
handleClick = (e)=> {
  .....
  this.setState({arr: newArr})
 //{arr: arr}
        
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