isInCognito.js

async function isInCognito() {
    const fs = window.RequestFileSystem || window.webkitRequestFileSystem;

    if (!fs) {
        return false;
    }

    let isInCognito = false;

    await fs(
        window.TEMPORARY,
        100,
        function(fs) {
            isInCognito = false;
        },
        function(err) {
            isInCognito = true;
        }
    );

    return isInCognito;
}