Destructuring causes error for null/undefined properties

The following code:

interface Foo {
    x: number;
}

interface Bar {
    foo?: Foo;
}

function test( { foo: { x } }: Bar ) {
    alert( x );
}

test( {} ); // TypeError: Cannot read property 'x' of undefined

transpiles to:

function test(_a) {
    var x = _a.foo.x;
    alert(x);
}

instead of the (arguably) preferable:

function test(_a) {
    var x;
    if (_a.foo) x = _a.foo.x;
    alert(x);
}

The transpiled code does not check for the presence of optional property foo in the Bar interface. This leads to an error at invocation.

I can see three possible solutions:

1. Always check for member existence when transpiling destructuring statements.
2. Check for member existence only when an element is optional in the interface or type declaration.
3. New syntax for destructuring that can indicate members should be checked.

E.g.

function test( { foo?: { x } }: Bar ) {}

[danielbodart]

What’s the current status of this?

[pauldijou]

I think you also have the problem when destructuring arrays.

const [a, b, c] = ''.split('')

TypeScript will consider all three variables to be string while they should be string | undefined (because, they are totally undefined here)