react useEffect中没有依赖关系？

Question

in what case I shouldn't put the empty array [] as dependcies in react useEffect? like

React.useEffect(() => {
    if (true) {
      doStuff()
    }
    return () => null;
  });

Is above code even valid?

Answer 1

Your code is valid.

undefined is passed as dependencies and it means useEffect runs on every render. So if you want to do some work that shouldn't be run on every render (like adding event listener), you should give a dependency array.

Answer 2

just remember 3 rules:

useEffect(()=>{},[]) //this is like componentDidMount
useEffect(()=>{}) //this will run every render
useEffect(()=>{},[dep1, dep2]) //this will run if one on de dependency change