Some problems in function default parameters?

See this

let foo = 'outer';

function bar(func = x => foo) {
  let foo = 'inner';
  console.log(func()); 
}

bar(); //outer

I want to know why the output is 'outer' rather than 'inner'. I know JavaScript has lexical scope. This output makes me feels like that the function x => foo is defined out of function bar