You can't set default values in an interface, but you can accomplish what you want to do by using optional properties:

Simply change the interface to:

interface IX {
    a: string,
    b?: any,
    c?: AnotherType
}

You can then do:

let x: IX = {
    a: 'abc'
}

And use your init function to assign default values to x.b and x.c if those properies are not set.

TypeScript can tell that x is definitely not defined, because it is a block-scoped variable and you can see the whole block.

If you know better than the compiler, you can separate the lines:

const def_val = 'default';

let x: string;
x = (typeof x === 'undefined') ? def_val : x;

But you might want to consider how the block scoped variable could possibly be undefined in your case (perhaps your code is not quite as simple as the example in your question).

The common use case would be more like:

const def_val = 'default';

function work(y: string) {
    let x = (typeof y === 'undefined') ? def_val : y;
}

You can also add more strict compiler options to make it less likely the value will be undefined in many cases.

Shorthand

There is also a shorthand falsey-coalesce that may be useful:

const def_val = 'default';

function work(y: string) {
    let x = y || def_val;
}

This will replace undefined, null, 0 (zero) or '' with the default.