How to use the application.start function in application

var application = require("application");
application.setCssFileName("./app.css");
application.start("main-page");

private bootstrapApp() {
    global.__onLiveSyncCore = () => this.livesyncModule();

    const mainPageEntry = this.createNavigationEntry(this._bootstrapper);

    application.start(mainPageEntry);
  }

insertBefore(newNode, referenceNode) {
        if (this.hasNonPlaceholderChild(this.view)) {
            throw new Error(`only one root is allowed.`)
        }
        if (newNode.type !== 'page') {
            throw new Error(`root node must be . <${newNode.type}> given.`)
        }

        if (this.isRootElement) {
            this.isRootElement = false
            this.placeholder = newNode
        }

        start({
            create() {
                return newNode.view
            }
        })
    }
}