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node.jsasynchronoussynchronization
Jpaji Rajnish
Jpaji Rajnish

Reputation: 1501

How does node.js decide if a statement is treated async or not?

I've been trying to wrap my head around understanding what node.js is going to add to the event loop to be processed asynchronously and what just gets executed in order like a normal synchronous program. For instance, why does this program execute synchronously?

console.log(longExecution())
console.log('hello');

function longExecution(){
    var results = '';
    for (var i=0; i < 10000000; i++) {
        if (i%1000 === 0)
            results += 'x'
    }
    return results;
}

This is the output:

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hello

I'm assuming if these statements were getting executed asynchronously that the the 'hello' would output before the 100,000 x's due to the increased processing time. But this isn't the case. So what criteria does node.js use to decide if a statement gets executed synchronously or asynchronously?

Upvotes: 2

Views: 591

Answers (1)

jfriend00
jfriend00

Reputation: 707158

node.js doesn't decide if something is sync or async. It's up to the underlying implementation to decide if the operation is coded to be sync or async. For example, in the file system APIs for node.js, there are both sync and async implementations and the programmer chooses which one to use. Those two implementations are coded differently in how they interface with the OS giving you either the synchronous or asynchronous implementation.

Regular javascript code such as your loop that isn't calling out to externally implemented functions or external libraries that might use threading or multiple processes is always synchronous in node.js so a for loop like you have will always be synchronous.

The kinds of things that can be asynchronous in node.js are things like network operations and file system operations or timers. Of course, it is possible using threading libraries or external processes to create your own function that may have asynchronous behavior, but again, it is the implementation of the function itself that makes it synchronous or asynchronous (e.g. how it's written).

Upvotes: 2

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