为什么这个递归捕获有时会为 .then 参数未定义？

Question

I'm new in debugging async code, and I get lost in the debugger flow. Can you spot why the greet parameter in .then(greet) is sometimes undefined?

 var tomsGreet = "Hello" var thinkSomething = (idea) => { console.log("I'm thinking") return new Promise((fulfill, reject)=>{ if(Math.round(Math.random() * 1)) { fulfill("Hello Tom, how is your " + idea) } else{ reject("Error found") } fulfill("Hello Tom, how is your " + idea) }) } var respondToTom = (response) => { return this.thinkSomething("dog") .catch((error) => { console.warn(`Error in thinking`) if (error === 'Error found') { console.log(`Retrying thinking`) return this.respondToTom(this.tomsGreet) } else { throw error; } }) .then((greet) => { if (greet == undefined){ console.error("Ups undefined") } console.log(greet) }); } this.respondToTom(tomsGreet);

Answer 1

Change to put the .catch after .then

var respondToTom = (response) => {
    return thinkSomething("dog")
    .then((greet) => {
        if (greet == undefined){
            console.error("Ups undefined")
        }
        console.log(greet)
    })
    .catch((error) => {
        console.warn(`Error in thinking`)
        if (error === 'Error found') {
            console.log(`Retrying thinking`)
            return respondToTom(tomsGreet)
        } else {
            throw error;
        }
    })
}

Answer 2

When you recursively call responseToTom() from the .catch() handler, you are relying on the fact that responseToTom() will return a promise that resolves to a value. But, in this code:

    .then((greet) => {
        if (greet == undefined){
          console.error("Ups undefined")
        }
        console.log(greet)
    });

You aren't returning any value so that will always resolve to an undefined value. You probably want to add return greet to that .then() handler.