EJS中JavaScript函数返回undefined：地址状态校验请求异常

The core problem here is that your is_address_exist function uses an asynchronous XMLHttpRequest, but you're trying to get a return value synchronously. The function finishes running right after calling xmlhttp.send(), long before the server responds — so it returns undefined by default, since there's no explicit return statement that waits for the request to complete.

Why Your Current Code Fails #

Looking at your snippet, the onreadystatechange handler only runs after the request finishes, but by that point, the is_address_exist function has already exited. There's no way to "return" the result from that async handler back to the original function call directly.

Solution 1: Use a Callback Function #

One classic way to handle async operations is to pass a callback function that gets executed once the request completes:

function is_address_exist(address, callback) {
  var xmlhttp = new XMLHttpRequest();
  var url = '../withdrawn/address_check/' + address;
  
  xmlhttp.open('GET', url, true);
  xmlhttp.send();
  
  xmlhttp.onreadystatechange = function() {
    // Wait for the request to fully complete (readyState 4) and succeed (status 200)
    if (xmlhttp.readyState === 4) {
      if (xmlhttp.status === 200) {
        // Trim whitespace to avoid unexpected results
        const addressStatus = xmlhttp.responseText.trim();
        // Pass the result to the callback
        callback(null, addressStatus);
      } else {
        // Handle request errors by passing an error to the callback
        callback(new Error(`Request failed with status: ${xmlhttp.status}`));
      }
    }
  };
}

// How to use it:
is_address_exist("your-target-address", function(error, status) {
  if (error) {
    console.error("Failed to check address:", error);
    return;
  }
  console.log("Address status:", status); // Will log 'yes' or 'no'
});

Solution 2: Use Promises (Modern Approach) #

For cleaner code (especially if you're working with other async operations), wrap the request in a Promise. This lets you use async/await for even more readable code:

function is_address_exist(address) {
  return new Promise((resolve, reject) => {
    var xmlhttp = new XMLHttpRequest();
    var url = '../withdrawn/address_check/' + address;
    
    xmlhttp.open('GET', url, true);
    xmlhttp.send();
    
    xmlhttp.onreadystatechange = function() {
      if (xmlhttp.readyState === 4) {
        if (xmlhttp.status === 200) {
          const addressStatus = xmlhttp.responseText.trim();
          resolve(addressStatus); // Resolve with the result
        } else {
          reject(new Error(`Request failed with status: ${xmlhttp.status}`)); // Reject on error
        }
      }
    };
  });
}

// Using async/await:
async function checkAddressStatus() {
  try {
    const status = await is_address_exist("your-target-address");
    console.log("Address status:", status); // Will log 'yes' or 'no'
  } catch (error) {
    console.error("Failed to check address:", error);
  }
}

// Call the async function
checkAddressStatus();

Important Notes #

• If your backend returns JSON instead of plain text (e.g., {"status": "yes"}), replace xmlhttp.responseText.trim() with JSON.parse(xmlhttp.responseText).status.
• Always handle request errors (like 404 or 500 status codes) — otherwise, you'll get silent failures if the server doesn't respond correctly.
• Ensure there are no CORS issues if your frontend is hosted on a different domain than the backend.

内容的提问来源于stack exchange，提问作者coin-o-pedia c