es6中的默认值不适用于箭头功能

Question

handleSuccessFeatureListing = (selectedOption=7) => {
    console.log(selectedOption);
}

Why selectedOption still can be null? I thought I already set 7 as the default value for the param of selectedOption?

Answer 1

Default values only come into affect if the function is called with no argument, or with an undefined value. If handleSuccessFeatureListing is called with null , null will be passed through.

eg

function fn(arg = 7){
  return arg;
}

fn() === 7
fn(undefined) === 7
fn(6) === 6
fn(null) === null

so if you are getting a null , then it is because null is being passed to the function when you expected an undefined value.