While reading through javascript codes I've been seeing the !
operator used for non boolean variables. Here is an example of code not used in.
/**
* loads a resource from a url
* @param {string} url the url of the resource to load
* @param {string} relativeTo the url to load relative to
* @param {function} callback thefunction to call once the file is loaded
* @private
*/
GLGE.Wavefront.prototype.loadFile=function(url,relativeTo,callback){
if(this.relativeTo && !relativeTo) relativeTo=this.relativeTo; //<-- used on a string?
else this.relativeTo=url;
if(!callback) callback=this.loaded; //<-- used on a function?
var req = new XMLHttpRequest();
if(req) {
// request handling code
}
};
req.open("GET", url, true);
req.send("");
}
}
In this library I've seen many uses of this operator in this manner.
Can someone explain how/if the 'not' function of a string, object or function can be determined when it isn't one half of a Boolean set like the set; true
and false
?
Best Answer
Any falsy value will satisfy the
if(!insert_variable_here)
condition, including:false
null
undefined
''
0
NaN
If
callback
return evaluates any of those values, the condition will be satisfied.Even though
null != false
, the following will give you an alert:Regardless of whether or not
null != false
makes sense to you or anyone else, the point is that in JavaScriptnull
is a falsy value, and thus a value that would satisfy the condition in my first bit of code listed above. This, it seems, is the question you have asked--not, rather, ifnull
should or should not== false
.