最后一次赋值后闭包中保留收窄

// *** before: ***
function getUrl(url: string | undefined) {
    if (url === undefined) {
        url = "https://example.com";
    }
    return () => {
        console.log(url.toUpperCase()); // <-Error: inside the closure url is
        //string | undefined again
    };
}

// *** in version 5.4: ***
function getUrlNarrowed(url: string | undefined) {
    if (url === undefined) {
        url = "https://example.com";
    }
    return () => {
        console.log(url.toUpperCase()); // OK: TS sees that url is never
        //reassigned
    };
}