Passing Parameters to KeyDown

问题:

How can you pass parameters into the keydown method from jquery because whenever I use a variable defined elsewhere it returns undefined. I assume its because the #target is window and therefore its not in the scope but even then I have trouble getting it to compare the key.which() with an outside parameter and then assigning it to another property.

Example:

var a = 38;
var b = 0;
$(document).keydown(function(e){
  var key = e.which;
  if (a==key)
    b=key;
});
console.log(a+""+b);

Whenever I try to do something along the same lines it returns 38 0 which I interpreted as it not being in the scope and being undefined (also because if I log b it prints undefined in the keydown func)? How could I pass in a?

回答1:

You console.log is not working because it is initialized when the script load. What you need to do is to trigger your functions when the key is pressed.

// When the document is ready
$(function() {
   $(document).keydown(function(e){
      var key = e.which;
      var a = 'a scoped variable';
      switch (key) {
          // Up arrow
          case 38: 
             a = upArrowFunction(a); // Assign a to the returned value.
             break;
          // Down arrow
          case 40:
             downArrowFunction();
             break;
      }

   });

   function upArraowFunction(a) {
       a = 'I change a, but a is not changed in keydown event';
       return a; // But now I return the changed variable so it will change if a variable is assigned where the function is called.
   }

   function downArrowFunction() {
       // Do something else
   }
});

回答2:

Do did you mean to have the console.log inside the keydown function?

I was able to get it to respond (as I would expect) like this:

$(function() { /* on page load load */
  var a = 38;
  var b = 0;
  $(document).bind('keydown',function(e){
    var key = e.which;
    if (a === key) {
      b=key;
    }
    console.log(a, b, key);
  });    
});

within the scope of the keydown function, a and b get set properly if you hit the up-arrow.

I suspect that what was happening was that your console.log happened on page load. Then after that, you initialized the binding and so you never saw the updated results.

回答3:

Try this

    $(document).on("keydown", function(e, key){
      var a = 38;
      var b = 0;
      if (e.which === a) {b = e.which};
      var key = (key === undefined ? b : a);
      return console.log(a + " " + b + " " + key)
    });

Edit

yes but the problem is that within key down I can't use a and b because they get set to undefined and I have to initiate them in another part of my program – user3030188

Call your functions when the keydown event is triggered and maybe you could send the variable to the function instead of making it global. – L105

Yah that makes sense but what im wondering is whether I could pass in a or b as a case statement if I defined them in another part of the program would they get overridden why would it return undefined? – user3030188

Try this

function key(e, a, b, key) {
/* settings */
   var a = (a === undefined ? 38 : a);
   var b = (b === undefined ? 0 : b);
   if (e.which === a) {b = e.which};
   var key = (key === undefined ? b : a);
   return console.log(a + " " + b + " " + key)
 };

$(document).on("keydown", key); /* `38` `38` `38` */

/* `a`, `b` */
$(document).on("keydown", key({},undefined,undefined,undefined)); /* `38` `0` `0` */

/* a`, `b`, or `n` */
$(document).on("keydown", function(n) {
  var n = 35;
  key({},37,n,undefined)
}); /* `37` `35` `35`*/

/* `n` */
$(document).on("keydown", function(n) {
  var n = 35;
  key({},n,n,undefined)
}); /* `35` `35` `35`*/