Example

When compiled in mode, optional properties and methods automatically have undefined included in their type. Thus, the b property above is of type number | undefined and the g method above is of type .Type guards can be used to strip away the undefined part of the type:

function test(x: Bar) {
    x.b;  // number | undefined
    let f1 = x.f();            // number
    let g1 = x.g && x.g();     // number | undefined