用于使用默认值解构函数参数的flowType

Question

Do we have any workaround in this situation that does not break type checking?

// Error
function foo({name = 42}: {name: ?number}) {
  //          ^ null or undefined [1] is incompatible with number [2].
  console.log(name);
}

The solution from the bug prevent flow from checking default value type.

// Correct
const foo = ({name = true}: $Subtype<{name: ?string}>) => {
  //                 ^ flow does not check this
  console.log(name);
}

Answer 1

The only workaround that also maintains type safety, is to not use destructuring:

function foo(props: {name: ?number}) {
    let name = props.name === undefined ? 41 : props.name;
    console.log(name);
}

Answer 2

We have an old code base for adding a types, so we want not to change the functionality.

Destructuring argument in the body instead of header - works great for me.

function foo(arg: {name: ?number}) {
  const { name = 42 } = arg;
  // the default value is only used for undefined values.
  console.log((name: (number | null)));
}