socket.on内部未定义的套接字

Question

I have the current code;

socket.on('KeyPress', function(data){
    var ply = PLAYER_LIST[socket.id];
    /* ... */
 });

WebStorm is informing me that socket.id may not be initialized, and then when this event is triggered it does cause the error

TypeError: Cannot read property 'id' of undefined

The whole code is inside of

io.sockets.on('connection', function(socket){ /*...*/ });

Here is two of the on method I'm using; In the first block you can see I'm doing the same thing yet it works..

Also on that note how secure is it to use socket.id to auth a user? Is it possible to force-set your own id?

Answer 1

You are a victim of a bad practice of defining variables with var within a loop and then getting bit by the influence of "variable hoisting" where a variable defined with var anywhere in the function is automatically declared at the start of the function and then initialized where your assignment was, meaning it's undefined everywhere in your function before that.

The problem is this line of code:

var socket = SOCKET_LIST[i];

That is redefining a local variable named socket that hides the one you actually want. Variables defined with var are function scoped. That means they are defined at the top of the function (this is called variable hoisting) and thus hides the other parent scoped socket variable.

Think of your function as starting like this:

socket.on('KeyPress', function(data) {
    var socket;       // this is hoisted from deeper inside the function
                      // and creates a new undefined socket variable
    var ply = PLAYER_LIST[socket.id];

    // other code here
});

This is essentially how Javascript sees your code. You can do a search on "Javascript variable hoisting" and see LOTS of articles written on the topic if you want more info.

The smallest change you can make it to change the name of this local variable to something else to it doesn't interfere with the parent scoped variable, but really this code structure is not ideal in the first place.

var localSocket = SOCKET_LIST[i];
var ply = PLAYER_LIST[p];
localSocket.emit(...)

---

In general it's a bad practice to define a variable with var inside a for loop. It leads to the "assumption" that it is scoped only to that loop, but in reality, it's scoped to the whole function and can mess with things outside of the loop.

In ES6, you can use let or const instead of var to declare a variable that is actually scoped to only the block of the loop.

Of other note, you should NEVER be iterating arrays with for/in . That iterates all array properties (including other enumerable object properties), not just array elements. ES6 adds for/of for iterating arrays this way. In ES5, you can use the more traditional for loop or .forEach() .