Enum type mutation for number, but not for string?

when I define an enum like this:

enum Dir {
    Up: 1,
    Down: 2
}

And then initialize a variable with its type cast to the enum Dir:

let dir: Dir;
dir = 3

I get no error. But when I make the enum string type like so,

enum Dir {
    Up: 'Up',
    Down: 'Down'
}

And then,

let dir: Dir;
dir = 'Left';

I get a compilation error. But in both cases, I assigned a value to dir variable that was not present in the enum. Why different behaviors for number and string types?