score:2

Accepted answer
addmistake(event){
   this.setstate((prevstate) => ({mistake: prevstate.mistake.concat([this.state.value]))
}); 
   console.log("mistake is "+ this.state.mistake.length + " and value is "+ this.state.value);
}

in render method

 render(){
  return(
    <div>
     <input value={this.state.value} type='text' onchange={this.handlechange} /> 
     <button onclick={this.addmistake}> add mistake </button>
      <div>
       {this.state.mistake.length && this.state.mistake.map(item =><p> {item} </p>) }
      </div>
       <div>{this.state.value}</div>
     </div>
        );
    }

this.state.mistake.length checks if we have anything in the array.

score:0

following syntax worked for me! thank you @omar and @hamms

addmistake(event){
    this.setstate({mistake: this.state.mistake.concat([this.state.value])
 }); 
    console.log("mistake is "+ this.state.mistake.length + " and value is "+ this.state.value);
 }

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