setState not working in React JS

Question

I am trying to give few dates to state.periods array. But it is not working. My code is as follows.

class SmallTable extends Component {
    constructor(props) {
        super(props);
        console.log(props.startDate)
        this.state = {
            turns: [],
            periods: []
        }
    }

componentDidMount() {
    //calculate years/ months and keep in one array
         const today = new Date();
         var periods1 = [];
         if (this.props.period=="year") { //if year calculate from last year last date
             const lastYearLastDate= new Date(new Date().getFullYear()-1, 11, 31)
             periods1.push(lastYearLastDate.getFullYear()+"-"+(lastYearLastDate.getMonth()+1)+"-"+lastYearLastDate.getDate());

             var lastYearFirstDate= new Date(lastYearLastDate.getFullYear()-1,0,1);

             //for the remaining periods
             for (var i=0;i<this.props.numberOfPeriods-1;i++) {
                 periods1.push(lastYearFirstDate.getFullYear()+"-"+(lastYearFirstDate.getMonth()+1)+"-"+lastYearFirstDate.getDate());
                 lastYearFirstDate = new Date(lastYearFirstDate.getFullYear()-1,0,1);
             }


         }


        else if (this.props.period=="month") {//if month calculate from last month last date
            var d=new Date(); // current date
            d.setDate(1); // going to 1st of the month
            d.setHours(-1); // going to last hour before this date even started.
            var lastMonthLastDate = d;
            periods1.push(lastMonthLastDate.getFullYear()+"-"+(lastMonthLastDate.getMonth()+1)+"-"+lastMonthLastDate.getDate());

            //go to last month  first date
            var lastMonthFirstDate = new Date(lastMonthLastDate.getFullYear(), lastMonthLastDate.getMonth(),1);

            //for the remaining periods
            for (var i=0;i<this.props.numberOfPeriods-1;i++) {
                periods1.push(lastMonthFirstDate.getFullYear()+"-"+(lastMonthFirstDate.getMonth()+1)+"-"+lastMonthFirstDate.getDate());

                lastMonthFirstDate=new Date(lastMonthFirstDate.getFullYear(), lastMonthFirstDate.getMonth()-1,1);
            }

        }

console.log(periods1); -->prints  ["2017-12-31", "2016-1-1", "2015-1-1", "2014-1-1"]
        this.setState((prevState)=>{
            return {
                periods: prevState.periods.push(periods1)
            }
        });
        console.log(this.state.periods) --> prints []

    }
render() {

        return ( <div></div>)
}

How to get values in periods1 to periods state. I am trying to insert periods1 array into state periods array. Those are strings. Pls suggest where the error might be.

Answer 1

You're setting this.state.periods to the result of a push operation. But push returns the new array length, not the array itself. Try this instead:

periods: [...prevState.periods, periods1]

Answer 2

push() doesn't return a value. You should use:

this.setState((prevState)=>{
    let old = prevState.periods.slice();
    old.push(periods1);
    return {
        periods: old
    }
});