Question

What's the output?

const obj = { 1: 'a', 2: 'b', 3: 'c' };
const set = new Set([1, 2, 3, 4, 5]);

obj.hasOwnProperty('1');
obj.hasOwnProperty(1);
set.has('1');
set.has(1);

• A: false true false true
• B: false true true true
• C: true true false true
• D: true true true true

Answer 1

Answer: C

All object keys (excluding Symbols) are strings under the hood, even if you don't type it yourself as a string. This is why obj.hasOwnProperty('1') also returns true.

It doesn't work that way for a set. There is no '1' in our set: set.has('1') returns false. It has the numeric type 1, set.has(1) returns true.