What is this snippet of JavaScript doing?

Question

I don't understand what each line is doing in the following snippet of JavaScript. I understand that it's reading whether the person pressed enter or escape. But I want to understand what line 2, 3, 6, 10, 16, and 18 are doing.

var okCancelEvents = function (selector, callbacks) {
  var ok = callbacks.ok || function () {};
  var cancel = callbacks.cancel || function () {};

  var events = {};
  events['keyup '+selector+', keydown '+selector+', focusout '+selector] =
    function (evt) {
      if (evt.type === "keydown" && evt.which === 27) {
        // escape = cancel
        cancel.call(this, evt);

      } else if (evt.type === "keyup" && evt.which === 13) {
        // blur/return/enter = ok/submit if non-empty
        var value = String(evt.target.value || "");
        if (value)
          ok.call(this, value, evt);
        else
          cancel.call(this, evt);
      }
    };

  return events;
};

Cheery · Accepted Answer · 2014-10-04 20:48:18Z

2

var ok = callbacks.ok || function () {};

if callbacks.ok is undefined, creates a stub function. http://en.wikipedia.org/wiki/Method_stub and sets ok to store reference to the that function or to reference in callbacks.ok

var cancel = callbacks.cancel || function () {};

the same for callbacks.cancel

events['keyup '+selector+', keydown '+selector+', focusout '+selector]

creates object events (previous line) and populates its attribute 'keyup '+selector+', keydown '+selector+', focusout '+selector with function.

cancel.call(this, evt);

calls function by reference stored in cancel variable, with listed arguments. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call

ok.call(this, value, evt);

the same, but for function reference in ok variable.

answered Oct 4, 2014 at 20:48

Cheery

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IMSoP · Accepted Answer · 2014-10-04 20:56:35Z

A good source of JavaScript reference material is the Mozilla Developer Network.

To pick out the lines you mentioned, with links to relevant MDN pages:

Lines 2 and 3 are using the || operator, which returns "something OR something else". In this case, it is being used to supply default values if either callbacks.ok or callbacks.cancel is undefined, so that the rest of the code can know that ok and cancel point at a valid function.
Line 6 is just concatenating lots of strings together into one object key using the + operator, which performs addition when given numbers, but concatenation when given strings.
Lines 10, 16, and 18, are all invoking callbacks using the call method, which is a way of setting an appropriate value for this when calling a function which is not defined as a member of a particular object.

The overall effect is to create an object events, with one property, whose name lists selector multiple times, and whose value is a function which can be passed an event and decide which of two callbacks to execute.

Note that this function doesn't include any logic for actually attaching the event to any DOM elements; the structure of its return value is presumably the input for some other function which does that.

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