Typescript-在arrow函数中更新变量

我有这样一个函数:

private data: MyCustomDataObject;
private getData(): boolean {
    let val: boolean = true;   // variable declared
    this.service.callMethod().subscribe(data => {
        this.data = data;
        if (data == null) {
            val = false;   // val declared above is not updating
        } 
    });
return val; // this is still true, should be false
}

怎么会 val