ApplicationView.Value | value property

var value = Windows.UI.ViewManagement.ApplicationView.value;

function onLoad() {
    // Register for the window resize event
    window.addEventListener("resize", onSizeChanged);
}

function onSizeChanged() {
    // Get view state
    var currentViewState = ApplicationView.value;

    if (currentViewState == ApplicationViewState.fullScreenLandscape) {
        // Full screen Landscape

    } else if (currentViewState == ApplicationViewState.fullScreenPortrait) {
        // Full screen Portrait

    } else if (currentViewState == ApplicationViewState.filled) {
         // Filled

    } else if (currentViewState == ApplicationViewState.snapped) {
        // Snapped
    } 
}

...
document.addEventListener("DOMContentLoaded", onLoad, false);