Deep assign to target from source, also performs distinct concat.
const join = require("join-assign");
const source = {
letters: [
"c",
"d"
]
};
const target = {
letters: [
"a",
"b"
]
};
join.assign(target, source);
const a = [1, 2, 3, 4, 5];
const b = [4, 5, 6, 7, 8];
join.concat(a, b);