You can do a function like this with array.reduce()

const getZones = (array) => {
    return array.reduce((acc, curr) => {
        if (curr.zones) {
            return [...acc, ...curr.zones];
        }
        return acc;
    }, []);
}

• flatMap get zones in one array.
• remove undefined with filter(Boolean)

array.flatMap(x=>x.zones) // ['188', undefined, undefined, undefined, '190', '189', '188']
.filter(Boolean); // ['188', '190', '189', '188']

When learning, you should use a simple for loop. With the spread operator (...) you can break an array into individual components.

Use a combination of both to get the zones :

const array = [
        {userId: 'P282', zones: ['188']},
        {userId: 'P277', zones: []},
        {userId: 'P280'},
        {userId: 'P281'},
        {userId: 'P279'},
        {userId: 'U57', zones: ['190', '189', '188']},
      ];
      
      let zones = [];
      for(let i = 0 ; i < array.length;i++){
      if(Array.isArray(array[i].zones))
      zones = [...zones,...array[i].zones];
      
      }
      
      console.log(zones);