ESLint wish

const y = new Set([10,20,30]);
const z = new Set([7,8,9]);

[1,2,3,4].forEach(y.add, z); // oops you added to z instead

console.log([...y]);
console.log([...z]);

// can't fault those who prefer this. I mean this line, not javascript's this :D
[1,2,3,4].forEach(Set.prototype.add, y); 

console.log([...y]);
console.log('---');
console.log([...z]);

If you are used to other language that automatically pass the this to the callback, you might want to be more explicit in JavaScript on passing which function you want to be called instead

Wish ESLint has a warning for passing callback like in line 4

[10, 20, 30]
[7, 8, 9, 1, 2, 3, 4]
---
[10, 20, 30, 1, 2, 3, 4]
[7, 8, 9, 1, 2, 3, 4]