add() appends a new element to the end of a Set object.
adds every value, of any type, only once
const set = new Set();
const fn = () => {};
set.add(1);
set.add(1);
set.add(fn);
set.add({fn});
assert.equal(set.size, 2);is chainable
const set = new Set();
set.add.add;
assert.equal(set.has(2), true);call without params adds undefined
const set = new Set();
set.add
assert.equal(set.has(void 0), true);0, -0 and +0 are equal
const set = new Set();
set.add();
set.add();
assert.equal(set.has(+0), true);