Flattening an array is nothing but returning only selected items in the form of array . we can push only the needed value instead of the item . see the example below const utubers=[{ id : 1 , name : 'Test Ed' , sal : 5000 },{ id : 2 , name : 'Dev Ed' , sal : 5000 },{ id : 3 , name : 'Mr Beast' , sal : 2000 },{ id : 4 , name : 'Mr Boss' , sal : 2000 },{ id : 5 , name : 'MKBHD' , sal : 5000 }] const salGt2000ids = utubers.reduce(( ( tempArray,utuber )=> { // get an array with only whose sal is more than 2000 if (utuber.sal> 2000 ){ tempArray.push(utuber.id) } return tempArray; } ),[]) console .log(salGt2000ids); //[1, 2, 5]