var obj = [
   { key1: 'value1' },{ key2: 'value2' },{ key3: 'value3' },{ key4: 'value4' }
]

[key1,key2,key3,key4]

let obj = [
   { key1: 'value1' },{ key4: 'value4' },];

let keys = [].concat(...obj.map(Object.keys));

console.log(keys);