Javascript fs.readfile() 不保存到变量

Question

I am trying to use javascrtipt fs.readfile() , but I can't get it to store what it has read to a variable. Here is my code:

const fs = require('fs')

var importantFinalCode = "waiting..."

fs.readFile("\pvMLTest.txt", "utf8", 
function(err, data){
  importantFinalCode = JSON.parse(data)
})


do {
  console.log("...")
} while (importantFinalCode == "waiting...")

console.log(importantFinalCode)

For some reason, it just infinitely logs ... , even though the function says to set the variable importantFinalCode to what it is reading. Do I need to make it a global variable? If so, how?

By the way, here is the file it is reading ( pvMLTest.txt ):

[["1", "2", "3"], ["3", "2", "4"]]

Thanks.

Answer 1

The following code is an infinite loop that takes up all of the event loop - therefore your script never completes:

do {
  console.log("...")
} while (importantFinalCode == "waiting...")

You can put the console into fs.readFile callback to be executed after file reading complete:

const fs = require('fs')

var importantFinalCode = "waiting..."

fs.readFile("pvMLTest.txt", "utf8", 
function(err, data){
  importantFinalCode = JSON.parse(data)
  console.log(importantFinalCode)
})

If your file is really big and you want to see the progress of its loading then you can use streams to read it:

const fs = require('fs');
const sourceFile = 'pvMLTest.txt';

var importantFinalCode = "waiting...";
console.log(importantFinalCode);

var data = '';
const readStream = fs.createReadStream(sourceFile);

readStream.on('data', function(buffer) {
    console.log('...');
    data += buffer;
});
readStream.on('end', function() {
    importantFinalCode = JSON.parse(data)
    console.log(importantFinalCode);
});

Answer 2

By doing this:

fs.readFile("\pvMLTest.txt", "utf8", 
function(err, data){
  importantFinalCode = JSON.parse(data)
})

You are telling JavaScript to execute the callback function function(err, data){ later. Later means whens the data arrives, and it also means when the current callstack is empty. By doing this:

do {
  console.log("...")
} while (importantFinalCode == "waiting...")

You are maintaining the current callstack alive forever. In short, the current callstack waits for the data, but the data callback waits for the current callstack to be empty. This is a kind of deadlock.

Whatever you want to do with the data, it must go inside the callback.

I suggest you read about the JavaScript's runtime specification which will help you understand why your code doesn't work: https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop

Answer 3

Fixed your issue. This was running an infinite loop due to the scope of the callback function within fs.readFile . Once you added your txt file data to the variable within the readFile function, it belonged to that function.

The do / while loop was accessing your gloabally assigned variable, instead of the one within the function.

Moving your loop into the callback function, fixed the issue. But not sure why you would have let importantFinalCode = "waiting..." because as soon as the readFile function runs, it changes the value of importantFinalCode.

const fs = require('fs')

let importantFinalCode = "waiting..."

fs.readFile("\pvMLTest.txt", "utf8", function (err, data) {
    if (err) {
        console.log(err)
    }
    importantFinalCode = data

    do {
        console.log("...")
    } while (importantFinalCode === "waiting...");
    console.log(importantFinalCode)
})

If you wanted to present the user with "waiting..." you could use a for loop like:

const fs = require('fs')

for (let i = 0; i <= 10; i++) {
    let importantFinalCode = "waiting..."
    console.log(importantFinalCode)
}

fs.readFile("\pvMLTest.txt", "utf8", function (err, data) {
    if (err) {
        console.log(err)
    }
    importantFinalCode = data

    console.log(importantFinalCode)
})