Assigning console.log to another object (Webkit issue)

问题

I wanted to keep my logging statements as short as possible while preventing console from being accessed when it doesn't exist; I came up with the following solution:

var _ = {};
if (console) {
    _.log = console.debug;
} else {
    _.log = function() { }
}

To me, this seems quite elegant, and it works great in Firefox 3.6 (including preserving the line numbers that make console.debug more useful than console.log). But it doesn't work in Safari 4. [Update: Or in Chrome. So the issue seems to be a difference between Firebug and the Webkit console.] If I follow the above with

console.debug('A')
_.log('B');

the first statement works fine in both browsers, but the second generates a "TypeError: Type Error" in Safari. Is this just a difference between how Firebug and the Safari Web Developer Tools implement console? If so, it is VERY annoying on ~~Apple's~~ Webkit's part. Binding the console function to a prototype and then instantiating, rather than binding it directly to the object, doesn't help.

I could, of course, just call console.debug from an anonymous function assigned to _.log, but then I'd lose my line numbers. Any other ideas?

回答1:

First, if console is indeed undefined (as it is in browsers such as IE), you'll get an error. You should check it instead as a property of the global object, which is window in browsers. It's also a good idea in general to test a feature before using it, so I've added a test for the debug method.

Possibly the implementation of console.debug in Safari relies on its value of this being a reference to console, which will not be the case if you call it using _.log (this will instead be a reference to _). Having done a quick test, this does seem to be the case and the following fixes the problem:

var _ = {};
if (typeof window.console != "undefined"
       && typeof window.console.debug == "function") {
    _.log = function() {
        window.console.debug.apply(window.console, arguments);
    }
} else {
    _.log = function() { }
}

回答2:

I've been looking for a solution to this myself (this is how I found your question).

As Tim pointed out, webkit browsers (Safari, Chrome) rely on this being console in this case. Firefox, however, does not. So in FF you can reassign the function and preserve the line numbers (otherwise all logs look like they originated in the logging function, which is not very useful). The best way to check which browser you are is to do it and check the result. Here is how to check it (in Coffeescript):

# Check to see if reassigning of functions work
f = console.log
assignSupported = true
try
  f('Initializing logging...')
catch e
  assignSupported = false

Later, when you make the functions check assignSupported and act accordingly:

levels =
  ERROR: 1
  WARN:  2
  LOG:   3
  INFO:  4
  DEBUG: 6

log.setLevel = (newLevel) ->
  for label, level of levels
    if level > newLevel # Skip low levels
      continue

    name = label.toLowerCase()
    f = -> # Fallback - empty function. In Js: var f = function () {}
    if console?[name]
      if assignSupported
        f = console[name] # Wee, we'll have line numbers.
      else
        # Webkit need the this of console.log (namely, console)
        # preserved, so we use a wrapper.
        #
        # Calling console[name] within the returned wrapper
        # makes [name] a subject of the closure, meaning
        # that it's the last value in the iteration -
        # we need to preserve it.
        f = ((n) ->
          return (-> console[n].apply(console, arguments)))(name)
    log[name] = f

log.setLevel levels.DEBUG

The lines:

f = ((n) ->
  return (-> console[n].apply(console, arguments)))(name)

might look a bit odd. This is because name is the loop variable and is lexically bound, meaning that the value at the time of execution will be used, which will always be the last level. It compiles to this javascript (in case it's easier to read):

f = (function(n) {
  return (function() {
    return console[n].apply(console, arguments);
  });
})(name);

来源：https://stackoverflow.com/questions/2619187/assigning-console-log-to-another-object-webkit-issue

标签

javascript

logging

console

webkit

firebug