very-small-ie-detect.js

// ----------------------------------------------------------
// A short snippet for detecting versions of IE in JavaScript
// without resorting to user-agent sniffing
// ----------------------------------------------------------
// If you're not in IE (or IE version is less than 6) then:
//     ie === 0
// If you're in IE (>=6) then you can determine which version:
//     ie === 7; // IE7
// Thus, to detect IE:
//     if (ie) {}
// And to detect the version (after IE has been detected):
//     ie === 6 // IE6
//     ie > 7 // IE8, IE9 ...
//     ie < 9 // Anything less than IE9
// ----------------------------------------------------------

// GOAL: the smallest possible minified size (without using conditional compilation)

// While using `with` makes automatic minification difficult (if not impossible)
// due to the fact that the minifier doesn't necessarily know which identifiers
// reference properties of the specified object versus variables in the scope
// chain, I'm using it here and minifying manually.
with ( document.createElement("b") )
  // No curly braces needed for the `with` because only the following `for`
  // loop (a single statement) needs to execute inside the `with`.
  for (
    // Initialize the for loop by declaring the `ie` var, setting its value to
    // -1 (which will get incremented at least once).
    var ie = -1;
    // Increment `ie` and update the innerHTML of the element accordingly. If
    // the browser is IE with a version greater than `ie`, "1" will be written
    // into the element.
    innerHTML = "<!--[if gt IE " + ++ie + "]>1<![endif]-->"
    // Unfortunately, because testing innerHTML at the time of assignment is
    // inadequate, a separate expression must be used. The comma operator allows
    // both the preceding and the following expression to be evaluated, with
    // only the last expressions's result affecting the for loop's condition.
    ,
    // If non-IE (or a lower version IE), the innerHTML will be an empty string,
    // which gets coerced to 0 (falsy) by the + operator. Otherwise, it will be
    // "1" which gets coerced to 1 (truthy). Basically, as long as the innerHTML
    // is "1", continue looping.
    +innerHTML;
    // Because `ie` is incremented in the condition, no increment expression is
    // needed here!
  );

// (It should go without saying that if the var was named `i` instead of `ie`,
// a few more bytes could be saved, but I'm saying it anyways, because that's
// how I roll)

// Minified (111 chars):
with(document.createElement("b"))for(var ie=-1;innerHTML="<!--[if gt IE "+ ++ie+"]>1<![endif]-->",+innerHTML;);

// Version without `with` in case that's how you roll (112 chars):
for(var ie=-1,b=document.createElement("b");b.innerHTML="<!--[if gt IE "+ ++ie+"]>1<![endif]-->",+b.innerHTML;);