RxJS中Observable的do()函数未触发，求原因？附StackBlitz示例

Hey there! Let's break down the common reasons why your RxJS Observable's do() (which is renamed to tap() in RxJS v6+) isn't firing, along with fixes for each scenario:

• Observable 从未被订阅
  This is the #1 culprit! Observables are "cold" by default—meaning their internal logic (and any operators in the chain like do()) only run when someone subscribes to them. If you forget to call .subscribe(), your do() will never execute.

  // ❌ Won't work: No subscription triggers the observable
  const myObs$ = of(42).do(val => console.log("Value:", val));
  
  // ✅ Works: Add a subscription to kick off the stream
  myObs$.subscribe();
  

• 上游 Observable 没有发出任何值
  If your source Observable doesn't emit any data (like an empty stream, or an async operation that never completes/succeeds), do() has nothing to react to. For example:

  // ❌ Empty observable emits no values, so do() doesn't run
  const emptyObs$ = empty().do(val => console.log("This won't log"));
  emptyObs$.subscribe();
  

• 流中发生未捕获的错误
  If an error occurs somewhere in your operator chain and you don't handle it with catchError(), the stream terminates immediately. Depending on where the error happens, your do()'s next callback might never fire (though the error callback of do() will trigger if you've defined it).

  // ✅ The error callback of do() will fire here
  const errorObs$ = throwError("Oops!").do(
    val => console.log("This won't log"),
    err => console.log("Error caught in do():", err)
  );
  errorObs$.subscribe();
  

  Always add error handling to avoid silent stream termination.

• 使用了已废弃的操作符或版本不匹配
  In RxJS v6 and later, do() was officially replaced with tap(). If you're using a newer version and still trying to use do(), it might not work as expected (or throw an error). Switch to tap() instead, and make sure to import it from rxjs/operators:

  // ✅ RxJS v6+ correct usage
  import { of } from 'rxjs';
  import { tap } from 'rxjs/operators';
  
  const updatedObs$ = of(42).pipe(tap(val => console.log("Value:", val)));
  updatedObs$.subscribe();
  

• 操作符链顺序错误
  If do() comes after an operator that terminates the stream without emitting values (like take(0) or first() when no values match), it won't trigger. Double-check your operator order:

  // ❌ take(0) stops the stream before any values are emitted
  const badOrderObs$ = of(1, 2, 3).pipe(
    take(0),
    tap(val => console.log("This won't log"))
  );
  badOrderObs$.subscribe();
  

内容的提问来源于stack exchange，提问作者bhantol