How to merge two array of objects with reactjs?

I have a react-big-calendar, I want to fetch the events of this week from the backend and the other weeks from the local storage.

My code is :

componentDidMount() {
    fetch("url")
    .then(Response => Response.json())
      .then(data => {
        let evts = data;
        for (let i = 0; i < evts.length; i++) {
          evts[i].start = moment(evts[i].start).toDate();
          evts[i].end = moment(evts[i].end).toDate();
          this.state.evt1.push(evts[i])
        }                   
        this.setState({
          evt1: evts,
          prevEvents : evts
        })
      }) 
      console.log(this.state.evt1)
      const cachedHits = JSON.parse(localStorage.getItem('Evènements')) 
      console.log(cachedHits)
      for (let j = 0; j <cachedHits.length; j++) {
        cachedHits[j].start = moment(cachedHits[j].start).toDate();
        cachedHits[j].end = moment(cachedHits[j].end).toDate();
        this.state.evt2.push(cachedHits[j])
      }
    this.setState( {
      evt2: this.state.evt2
  })
    this.setState({
      events: [...this.state.evt1, ...this.state.evt2]
    })
  console.log(this.state.events)
  }

the events is the merged array of evt1 (events from the backend) and evt2 (events from the localstorage), when I run it, I get on my console :

The evt1 are :

The evt2 are :

But, on my calendar, just the evt2 are displayed and not all the events (evt1 and evt2).

How can display all the events on my calendar ?

Because you are making an HTTP call to your server, it takes some time to fetch the data. Instead of setting events state directly, you should wait for the response of your HTTP call. Your code should be like this:

componentDidMount() {
    fetch("url")
        .then(Response => Response.json())
        .then(data => {
            let evts = data;
            for (let i = 0; i < evts.length; i++) {
                evts[i].start = moment(evts[i].start).toDate();
                evts[i].end = moment(evts[i].end).toDate();
                this.state.evt1.push(evts[i])
            }
            this.setState({
                evt1: evts,
                prevEvents: evts
            })
        })
        .then(() => {

            console.log(this.state.evt1)
            const cachedHits = JSON.parse(localStorage.getItem('Evènements'))
            console.log(cachedHits)
            for (let j = 0; j < cachedHits.length; j++) {
                cachedHits[j].start = moment(cachedHits[j].start).toDate();
                cachedHits[j].end = moment(cachedHits[j].end).toDate();
                this.state.evt2.push(cachedHits[j])
            }
            this.setState({
                evt2: this.state.evt2
            })
            this.setState({
                events: [...this.state.evt1, ...this.state.evt2]
            })
            console.log(this.state.events)

        });
}

I also recommend you to have a catch block in your promise chain to handle errors.