Take this example
var s;
cb = function(error,result){
s = result;
}
memcached.get('hellow',cb);
console.log(s);
This give me undefined. What is wrong with my code?
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The answers on this question so far have been great; I wanted to mention that you may be interested in this screencast, which covers this exact issue: nodecasts.net/episodes/5-thinking-asynchronouslyMichelle Tilley– Michelle Tilley2012-09-02 17:59:42 +00:00Commented Sep 2, 2012 at 17:59
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3 Answers
The console.log(s); line executes before the cb function does because cb isn't called by memcached.get until result is available. This is the classic asynchronicity that occurs with any I/O operation in node.js.
3 Comments
Mostafa Darwish
yes, but how i could get the
s value outside callback function?
JohnnyHK
Not quite sure what you're asking. Any code that relies on the result of the
memcached.get must execute after the callback is called. So you can put your code that references s in the callback, or use a flow control library like async to execute another function after cb is called.
Golo Roden
To cut a long story short: You can't - at least not without additional helpers such as the already mentioned
async module.You need to execute console.log after defining s because it asynchronous:
var s;
cb = function(error,result){
s = result;
console.log(s);
}
memcached.get('hellow',cb);
2 Comments
Mostafa Darwish
good but is there is another way to get
s value outside callback function?
Vadim Baryshev
call external function, emit event or wait some time with setTimeout (not the best way)
The variable s is being initialized within a call back function. This call back function will get triggered only when memcached.get() completes getting the data for 'hellow'.
Javascript reliesy on the event loop mechanism. This means that the javascript runtime will continue executing the script until it reaches the end without blocking for any callback to occur.
As a result in your example, the javascript runtime will execute the line console.log(s) immediately after the line memcached.get('hellow',cb) without blocking. Hence console.log(s) in your case will print a valid value (other then undefined) only if the cb has executed prior to the last line.
Please move the line console.log(s) within the call back function for more consistent results.
1 Comment
Mostafa Darwish
it seams that this is the only way to achieve result
