const value = { number: 10 }; const multiply = (x = { ...value }) => { console.log((x.number *= 2)); };

Question

What's the output?

const value = { number: 10 };

const multiply = (x = { ...value }) => {
  console.log((x.number *= 2));
};

multiply();
multiply();
multiply(value);
multiply(value);

• A: 20, 40, 80, 160
• B: 20, 40, 20, 40
• C: 20, 20, 20, 40
• D: NaN, NaN, 20, 40

Answer 1

Answer: C

In ES6, we can initialize parameters with a default value. The value of the parameter will be the default value, if no other value has been passed to the function, or if the value of the parameter is "undefined". In this case, we spread the properties of the value object into a new object, so x has the default value of { number: 10 }.

The default argument is evaluated at call time! Every time we call the function, a new object is created. We invoke the multiply function the first two times without passing a value: x has the default value of { number: 10 }. We then log the multiplied value of that number, which is 20.

The third time we invoke multiply, we do pass an argument: the object called value. The *= operator is actually shorthand for x.number = x.number * 2: we modify the value of x.number, and log the multiplied value 20.

The fourth time, we pass the value object again. x.number was previously modified to 20, so x.number *= 2 logs 40.