NodeJS使用module.exports导出异步变量

Question

I have 2 files in a nodejs app:

export.js (reads a file line by line and saves it into an array)

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream('test.txt')
});
var file_lines = [];

onFinish(function(result) {
    module.exports = result;
});

function onFinish(callback){
    lineReader.on('line', function (line) {

        file_lines.push(line);

    }).on('close', function() {
        callback(file_lines);
    })
};

and app.js (should get the exported values so I can play with it in here)

var a = require("./export");

console.log(a);

As you can see in the export.js I've used an async callback so it will save the data, but I think I'll need another async callback in the app.js file. What would be the best way to achieve this?

PS I realized that the async call for the export.js file isn't necessary. Here is the new version:

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream('test.txt')
});
var file_lines = [];

lineReader.on('line', function (line) {

    file_lines.push(line);

}).on('close', function() {
    module.exports = file_lines;
    console.log(file_lines);
})

But again, my question will be, how would I get the values in the a variable in the app.js file?

Answer 1

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream('test.txt')
});
var file_lines = [];

module.exports = function onFinish(callback){
    lineReader.on('line', function (line) {

        file_lines.push(line);

    }).on('close', function() {
        callback(file_lines);
    })
};

and then

var a = require("./export");

a(function(data){ console.log(data) })

Answer 2

Instead of a callback, you can also export a promise :

var Promise = require('bluebird')

var lineReader = require('readline').createInterface({
    input: require('fs').createReadStream('test.txt')
});

module.exports = function (user) {
    return new Promise(function (resolve, reject) {
        var file_lines = [];

        lineReader.on('line', function (line) {

        file_lines.push(line);

        }).on('close', function() {
            resolve(file_lines);
        })

    })
}

Then use it nicely, like this:

read_lines = require('line_reader')

read_lines()
    .then(function (lines) {
        // lines are already read here
    })

to get the Promise, you can install bluebird with npm i bluebird

Answer 3

您可以导出功能lineReader并设置onFinish回调函数，其中console.log可以写