How to do something synchronous inside a asynchronous function?

Question

const fs = require('fs')
const readline = require('readline')
const stream = require('stream')

const rl = readline.createInterface({
    input: fs.createReadStream('logs_out.txt')
})

var items = new Set()

// ASYNC FUNCTION READING LINE BY LINE

rl.on('line', function(line) {
   let log = JSON.parse(line)
    // ADDING ITEMS TO A SET TO GET UNIQUE ITEMS COUNT
   items.add(log.resource)
})

// PRINTING OUT THE SIZE OF THE SET
console.log(items.size)

Now the issue is, when I do this I always get undefined or 0 for the size of the set because the console.log(items.size) is executed even before all the line functions are executed.

How to fix this.

Thank You.

Yes it looks like a possible duplicate with the additional complexity of having multiple asynchronous calls. — Subir Kumar Sao, Oct 08 '17 at 17:03
A clear answer would be really appreciated, as I have just started out in JS it is a little difficult for me to understand all the concepts in that linked answer. — deadpoolAlready, Oct 08 '17 at 17:04
@deadpool put the last line into sth like `rl.on("end",function(){ /*here*/ })` — Jonas Wilms, Oct 08 '17 at 17:13
@Jonasw Thanks mate, I used the close event and it worked, but any idea how to call something synchronous from async somewhere else where events like close are not available? — deadpoolAlready, Oct 08 '17 at 17:15

score 0 · Accepted Answer · answered Oct 08 '17 at 17:13

As noted as a possible duplicate by @str, there's a superb answer already here on Stack Overflow for why you might be best off with just embracing the asynchronous approach: How do I return the response from an asynchronous call?.

For your specific problem, I suggest you just use the close event from readline.

The readline.Interface instance should be considered to be "finished" once the 'close' event is emitted.

In your example, you can use the following:

// PRINTING OUT THE SIZE OF THE SET
rl.on('close', function() {
    console.log(items.size)
}

How to do something synchronous inside a asynchronous function?

1 Answers1