Update person field (multiple selection) using JavaScript without PnP

How to update a person field using spHttpClient

A few things to note:

1. Before doing the actual update you must ensure the user Id of the user, this is what you will input as the field value.
2. You must add Id to your field name. If the field name is Personyou should use PersonIdin the update body.

private updatePeopleField(username: string, listname: string, itemid: string) {
    let client = this.props.context.spHttpClient;
    let clientconfig = SPHttpClient.configurations.v1;
    let options:ISPHttpClientOptions = {
      body: JSON.stringify({'logonName' : username})
    };
    client.post("/_api/web/ensureuser", clientconfig, options).then((res:SPHttpClientResponse) => {
      res.json().then(json => {
        return json.Id;
      }).then(userid => {
        let body= JSON.stringify({
          '__metadata': { 'type': `SP.Data.${listname}ListItem` },
        'PersonId': userid});

        let updateoptions = {
          headers: {
            'Accept': 'application/json;odata=nometadata',
            'Content-type': 'application/json;odata=verbose',
            'odata-version': '',
            'IF-MATCH': '*',
            'X-HTTP-Method': 'MERGE'
            },
          body: body
        };
        client.post(`/_api/web/lists/getByTitle('${listname}')/items('${itemid}')`, clientconfig , updateoptions)
          .then((response: SPHttpClientResponse) => {
            console.log('Item updated!');
          });
      });
    });
  }