Nested Property Guards

NestedPropertyGuard.ts

interface InputArea {
    control: HTMLTextAreaElement | HTMLInputElement;
}

interface Component {
    inputArea: InputArea;
}

function setupComponent(component: Component) {
    if ('rows' in component.inputArea.control) {
        // These properties are only valid for HTMLTextAreaElement
        component.inputArea.control.rows = 25;
        component.inputArea.control.cols = 80;
        component.inputArea.control.wrap = 'hard';
    } else {
        // These properties are only valid for HTMLInputElement
        component.inputArea.control.width = 400;
        component.inputArea.control.height = 50;
    }
}