javascript - 为什么我的 promise 的 'resolve' 在函数结束执行之前被发送？

Question

我有一个函数(内部包含 promise ，因此它本身同步运行)似乎在我的主代码中异步运行。无论我如何格式化我的 promise ，似乎在函数结束执行之前发送了解决方案:

这个问题在逻辑上也是递归的，因为如果我尝试在 nameExists 函数周围添加另一个 promise(在这个 promise 内)，然后将 resolve 放在 'then' 中，我就会遇到与嵌套 resolve 相同的问题...

    document.getElementById("config-select").addEventListener("input", function(){
      //check if the doc name exists: returns doc id
      //promise that doc_obj is created before moving on
      let doc_obj = {};
      let promise = new Promise(function (resolve, reject) {
        let doc_name = document.getElementById("config-select").value;
        doc_obj = nameExists(doc_name);
        resolve('done'); //this executes BEFORE nameExists is done processing...bringing back the original asynch issue i was trying to fix in the first place...
      });
      promise.then(function (result) {
          alert("then: "+doc_obj);
          if(doc_obj.bool === true){//it does exist:
            alert("replacing id");
            document.getElementById("config-select").setAttribute("doc-id", doc_obj.id);
          }
          else{//it doesn't:
            alert("resetting id");
            document.getElementById("config-select").setAttribute("doc-id", "");
          }
        }
      );

    });

nameExists 函数:

//check if the name in config-select is an existing doc (assumes name is a unique document field)
const nameExists = function(name){
  //get all docs
  localDB.allDocs({include_docs: true}).then(function (result) {
    //return object set to default state if no match is found
    let doc_obj = {bool: false, id: ""};
    alert("Entering the match checker...");

    for(let i =0; i<result.total_rows; i++) {
      if(result.rows[i].doc.name == name){
        alert(result.rows[i].doc.name);
        alert(name);
        doc_obj.bool = true;
        doc_obj.id = result.rows[i].doc._id;
        //found a match
        break;
      }
    }
    //return the result
    alert("returned obj.id: "+doc_obj.bool);
    return doc_obj;

  }).catch(function (err) {console.log(err);});
};

理想情况下，在评估我的“if 语句”之前，我希望使用来自 nameExists 函数的数据填充 doc_obj 或某些返回值对象。我如何格式化我的 promise/resolve 语句来实现这一点？

Answer 1

你应该放弃那个 new Promise - 它不会改变你是否能够等待 nameExists 的结果。您将需要返回 then() 在nameExists 函数中创建的 promise :

function nameExists(name) {
  return localDB.allDocs({include_docs: true}).then(function (result) {
//^^^^^^
    for (let i =0; i<result.total_rows; i++) {
      if (result.rows[i].doc.name == name){
        return {bool: true, id: result.rows[i].doc._id};
      }
    }
    return {bool: false, id: ""};
  });
//  ^ don't catch errors here if you cannot handle them and provide a fallback result
}

然后你可以在你的事件监听器中等待它:

document.getElementById("config-select").addEventListener("input", function() {
  const doc_select = document.getElementById("config-select");
  const doc_name = doc_select.value;
  // check if the doc name exists: returns doc id
  nameExists(doc_name).then(function(doc_obj) {
//^^^^^^^^^^^^^^^^^^^^^^^^^^          ^^^^^^^
    console.log("then", doc_obj);
    if (doc_obj.bool) { // it does exist:
      alert("replacing id");
    } else { // it doesn't:
      alert("resetting id");
    }
    doc_select.setAttribute("doc-id", doc_obj.id); // id is "" when it doesn't exist
  }).catch(function (err) {
    console.log(err);
  })
});