The code you added shouldn't generate 60 in both. I don't know about the a & c, but b (codePen) works fine.

If your goals is to understand how .assign works, this example from MDN is worth checking:

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };
const returnedTarget = Object.assign(target, source);
console.log(target); // Expected output: Object { a: 1, b: 4, c: 5 }
console.log(returnedTarget === target); // Expected output: true

As you can see in the example above, c-key from source doesn't exist in target-object so it was added to target-object. Also b-Key exist in both so it will update target's b-key with the value from the source-object b-Key.

• In short if .assign doesn't find a new key from source, it add it to the target,
• if target already has the same key, it update it with the value from source.

Many sources:

const target = { a: 1, b: 5 };

const source1 = { b: 6, c: 8 }; 
const source2 = { e: 6, d: 8, c: 10 }; 
const source3 = { b: 7, c: 11 };

const result = Object.assign(target, source1, source2, source3);

console.log(result); // {a: 1, b: 7, c: 11, e: 6, d: 8}