node memcached have a function get to retrieve memcachd key value for e.g.:
memcached.get( "hello",function( err, result ){
if( err ) console.error( err );
this.r = result;
console.dir( result );
memcached.end();});
The second argument in the function is the callback function
inside the callback function I can log the value of my key easily.
I tried to assign a property r to memcached and when I try to log the memcached.r outside get function undefined result occurs.
What's wrong with my code?
Although this looks like a variable and often behaves like one, it isn't truly a variable and thus does not behave like one in closures. When you've got one function nested inside another, the inner function gets a completely new version of this which is very unlikely to point to the same thing as the outer function's this.
Therefore, if you need to refer to an outer function's this inside an inner function, you have to first copy it into a true variable (most people like to call it self but some prefer that) and then refer to that variable in the inner function.
In your case, however, you've already got a real variable, namely memcached, that you can assign to, so just replace this with memcached in your callback. It's a properly-scoped outer variable of the closure created by your callback (as can be seen by the fact that you can use memcached.end() in it).
[Update to include correctly formatted version of your code mentioned in comment]
memcached.get = function get(key, callback) {
if (Array.isArray(key)) return this.getMulti.apply(this, arguments);
this.command(function getCommand (noreply) {
return { key: key , callback: callback ,
validate: [['key', String], ['callback', Function]] ,
type: 'get' , command: 'get ' + key };
});
};
It looks like you're trying to call several functions asynchronously and expecting to return a value from them. That simply won't work.