如何从嵌套的 javascript function 返回一个值？

Question

window.RTCPeerConnection = window.RTCPeerConnection || window.mozRTCPeerConnection || window.webkitRTCPeerConnection;  
var pc = new RTCPeerConnection({iceServers:[]}), noop = function(){};      
pc.createDataChannel("");   
pc.createOffer(pc.setLocalDescription.bind(pc), noop);        
pc.onicecandidate = function(ice){ 
  if(!ice || !ice.candidate || !ice.candidate.candidate)  return;
  ice.candidate.candidate.split('\r\n').forEach(function (line) {   
    if (line.indexOf("candidate")!==-1) {           
      var parts = line.split(' '),             
      addr = parts[4],
      type = parts[7];
      if (type === 'host') {
        console.log(addr);
        return addr;
      }
    } 
  });
  pc.onicecandidate = noop;
};

In the above javascript, the console.log(addr) works but not the return. Please point me where I am doing wrong. I tried to wrap this in a function and that doesn't work it either.

Answer 1

As long as your code in synchronous, you can write variables outside of the immediate function scope like so:

let result = null;
ice.candidate.candidate.split('\r\n').forEach(function (line) {   
    if (line.indexOf("candidate")!==-1) {           
      var parts = line.split(' '),             
      addr = parts[4],
      type = parts[7];
      if (type === 'host') {
        console.log(addr);
        // Save value
        result = addr;
      }
    } 
  });
// ...
console.log(result);

This can be further improved by using a for (let.. of..) loop and break to stop the loop after finding the value you're looking for.

You should also consider Array.prototype.find() for this.

Answer 2

pc.onicecandidate = function(ice){ 
  if(!ice || !ice.candidate || !ice.candidate.candidate)  return;

  const hosts = [];

  ice.candidate.candidate.split('\r\n').forEach(function (line) {   
    if (line.indexOf("candidate")!==-1) {           
      var parts = line.split(' '),             
      addr = parts[4],
      type = parts[7];
      if (type === 'host') {
        console.log(addr);

        hosts.push(addr)

      }
    }
  });
  pc.onicecandidate = noop;
  
  return hosts;
};

Is something like that what you are after?

You want to return each host address?

This stores the hosts locally. Foreach is synchronous so when it has completed working on the collection then your function will reach the return statement.

If you want to find the first address then you might want to use.find instead of.forEach, or check that a value exists and bail from the forEach when it does (note that you're still calling that function on each item, it'll just return straight away). You can break out of a forEach but its not idiomatic, you'd have to throw an exception and you try..catch, a bit nasty in JS and not really the intended use for the syntax.

pc.onicecandidate = function(ice){ 
  if(!ice || !ice.candidate || !ice.candidate.candidate)  return;

  const line = ice.candidate.candidate.split('\r\n').find(function (line) {   
    if (line.indexOf("candidate")!==-1) {           
      var parts = line.split(' ');
      var type = parts[7];
      return type === 'host'
    }
  });
  pc.onicecandidate = noop;
  return line.split(' ')[4]
};

Something like this I think should work for.find. .find will return the item, which you can then pull the addr from. Alternatively you could let addr outside, update that inside, then ignore the return value from.find. Oh, you'll want to add something if your.find returns nothing, or things will break when you try to split it and pull something out.