获取回调以等待异步调用

编程入门行业动态更新时间:2024-10-07 16:17:01

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获取回调以等待异步调用

我对所有这些东西都很陌生，有几个关于 Nodejs/js 中的异步和回调的问题。

我有以下小提琴设置：/

function updateTestVariable(key, callback) {
  console.log("2 ENTERupdateTestVariable key is: " + key);
  callback(key)
}

var testVariable = ""

console.log("1 BEFORE updateTestVariable is called testVariable is " + testVariable)

updateTestVariable("VARIABLE HAS UPDATED", async function(firstParam) {
  console.log("3 ENTER updateTestVariable firstParam is: " + firstParam);
  console.log("4 text before timeout starts testVariable is: " + testVariable)

  var mockingAsyncServiceCall = await setTimeout(() => {
    testVariable = firstParam
    console.log("5 AFTER TIMEOUT COMPLETE  testVariable is " + testVariable)
  }, 4000);

  console.log("6 line after timeout, testVariable is " + testVariable)
})

console.log("7 AFTER updateTestVariable is called testVariable is " + testVariable)

回答如下：

要让 5 在 4 之后出现，请考虑将

setTimeout()

包裹在

Promise

中。这允许回调正确地“等待”

setTimeout()

解决，然后调用 6。

对于最后一个 7，在

callback

中返回

updateTestVariable()

的调用结果，这样

updateTestVariable()

就会返回一个

Promise

，从而我们可以使用

.then()

仅在

updateTestVariable()

之后记录 7彻底解决

function updateTestVariable(key, callback) {
  console.log("2 ENTERupdateTestVariable key is: " + key);
  return callback(key)
}

var testVariable = ""

console.log("1 BEFORE updateTestVariable is called testVariable is " + testVariable)

updateTestVariable("VARIABLE HAS UPDATED", async function(firstParam) {
  console.log("3 ENTER updateTestVariable firstParam is: " + firstParam);
  console.log("4 text before timeout starts testVariable is: " + testVariable)

  var mockingAsyncServiceCall = await new Promise((resolve) => {
    setTimeout(() => {
      testVariable = firstParam
      console.log("5 AFTER TIMEOUT COMPLETE  testVariable is " + testVariable);
      resolve();
    }, 4000);
  });

  console.log("6 line after timeout, testVariable is " + testVariable);
}).then(() => console.log("7 AFTER updateTestVariable is called testVariable is " + testVariable));

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获取回调以等待异步调用

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