Используя forEach как вы, но for in
внутри.
var objArray = [{foo: ["f", "b"],bar: ["e"]},{foo: ["a", "c"],bar:["a", "c"]},{foo: ["f", "b"],bar: ["z"]}];
var newObj = {};
objArray.forEach(o => {
for (p in o) {
if (!(p in newObj)) {
newObj[p] = [];
}
newObj[p] = [...newObj[p], ...o[p]];
}
});
console.log(newObj);