I was looking in the javascript reference manual on the indexOf page at developer.mozilla.org site, and noticed a few things in their implementation code of indexOf, I hope somebody can explain to me.
To save everybody a round trip to the mozilla site, here is the entire function:
if (!Array.prototype.indexOf)
{
Array.prototype.indexOf = function(elt /*, from*/)
{
var len = this.length >>> 0;
var from = Number(arguments[1]) || 0;
from = (from < 0)
? Math.ceil(from)
: Math.floor(from);
if (from < 0)
from += len;
for (; from < len; from++)
{
if (from in this &&
this[from] === elt)
return from;
}
return -1;
};
}
What I do not understand is the /*, from*/
in the function declaration, and the zero-fill right shift >>>
in the extracting of the length of the array (var len = this.length >>> 0;
).
The /*, from */
is a commented out parameter. However it looks like it has been left in the comments to show that this parameter can optionally be specified for the function.
var from = Number(arguments[1]) || 0;
I believe that arguments[1]
would be the from
value if passed in.
The arguments array is especially useful with functions that can be called with a variable number of arguments, or with more arguments than they were formally declared to accept. http://www.devguru.com/Technologies/Ecmascript/Quickref/arguments.html
The >>>
is an unsigned right shift. It's being used here to convert a potentially signed number length
into an unsigned number.
Extract from Professional JavaScript for Web Developers
http://www.c-point.com/javascript_tutorial/jsoprurshift.htm
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