js2 mode doesn't recognize variable in "for in" statement

Question

When I use a for in statement, js2 mode doesn't recognize the new variable:

let foo = {bar: 1};

for(key in foo) {
    console.log(key);
}

It prints: Undeclared variable or function 'key'

Answer 1

It warns you that the key variable is undeclared, and using it like this, you leak it into the global namespace.

Try this is your browser console:

> let foo = {bar: 1};

> for(key in foo) {}

> key
=> "bar"