array.length is zero, but the array has elements in it

Question

I'm currently in the process practicing using electron, but I'm quite new with javascript and I've come across a problem which has me completely baffled. I have the following code:

    function getPaths() {
      var dirPath = document.getElementById("mdir").innerHTML;
      var filePaths = [];
      fs.readdir(dirPath, function(err, dir) {
        for(var i = 0, l = dir.length; i < l; i++) {
          var filePath = dir[i];
          filePaths.push(dirPath + "/" + filePath);
        }
      });
      console.log(filePaths);
      console.log(filePaths.length);
    }

Which is supposed to look into a directory defined by dirPath, then it loops through and obtains the full path of all files in that directory. It appends them to an array, and then at the bottom, it logs the array to the console, followed by the length of the array. What is baffling me is that given that code, the array logs to the console like expected, but then the console logs zero as the length. My current thinking is that it's got something to do with scope, but that doesn't make sense because I'm declaring the array, filePaths in the function above the one that's running. Unless I've missed something. Could anyone point out what I'm doing wrong?

ibrahim mahrir · Accepted Answer · 2021-07-25T14:43:22.680

44

readdir is asynchronous. It won't get the results right away. You should use the filePaths inside the callback. The only reason why the console shows the value is because the console evaluates the array when you unfold it.

When you press the little arrow on the left, put the mouse on the i box on the right. What happens is that the console keeps a reference to the array, so when the user unfolds the array it then shows the current value of the array. But when you log filePaths.length the array is empty because readdir didn't finish reading yet, that's why you get 0. But by the time you open the console and press that arrow, readdir will have already done reading and the console will print the current value of the array (after it's been filled).

Example to demonstrate the problem: (not a solution, it's just to understand what is really happening)

Open the browser console and try this code and see what happens:

var arr = [];

setTimeout(function() {
  arr.push(1, 2, 3);
}, 5000);

console.log(arr.length);

console.log(arr);

Here the array and it's length are both logged before the array is filled. The array will be filled after 5 seconds. So the output will be 0 and a string representation of the array array[]. Now because arrays could have tons of data, the console won't show that data until the user unfolds the array. So what the console does is keep a reference to the array until the user press the unfold arrow. If you unfold the array before 5 seconds you'll see that the array is empty (not filled yet). If you wait until the 5 seconds pass then unfold it, then you'll see that it's filled, even though it was logged as an empty array.

Note: Also, the line that get logged to the console (something like > Array(0)) is just a string representation of the object/array at the moment the log happens. It won't get updated if the object/array changes. So that also may seem confusing sometimes.

I hope it's clear now.

edited Jul 25 '21 at 14:43

answered Feb 15 '17 at 21:39

ibrahim mahrir

31,174
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2

I would not recommend to use `setTimeout` function as the response time could vary significantly based on the directory content – MaxZoom Feb 15 '17 at 22:51
5

@MaxZoom I didn't suggest `setTimeout` as a solution! I just explained the OP's problem using a easy-to-understand example using `setTimeout` to emphasize on the asynchronousity of the problem! – ibrahim mahrir Feb 15 '17 at 22:54
2

Ok, so if I've got this right, the reason why I'm not getting a value is because readdir is asynchronous. As a result, console.log() completes before the array has been built, and as a result it returns 0? – Taira Feb 15 '17 at 23:12
@Taira exactly! `readdir` will just regester the callback to be called when the reading is done, and proceed to the next statement and execute it (pretty much the same of what happening with `setTimeout)`)! If you want the synchronous version use `readdirSync` (but I wouldn't recommend it). – ibrahim mahrir Feb 15 '17 at 23:15
Ok, that makes sense! I come from a background in Python, so this type of thing is quite foreign to me. One other question though: out of curiosity, why would `readdirSync` not be recommended? – Taira Feb 15 '17 at 23:18
@Taira the best about NodeJS (the thing that electron uses) is the asynchronousity of its I/O methods. Why? Because I/O method could be slow sometimes, so it's not good to block everything waiting for the function to complete. The best approach is to regester a callback for the function to call once it's done reading. Imagine if the reading took 1 to 2 seconds (just imagine), then nothing else will be executed during that time. But if it's asynchronous, you could do a lot of stuff during that time. See the diference? – ibrahim mahrir Feb 15 '17 at 23:25
Yes, I do. Thank you! – Taira Feb 16 '17 at 00:54

score 1 · Answer 2 · answered Feb 15 '17 at 21:41

Just to expand on @ibrahim-mahrir 's answer, they means like this

function getPaths() {
    var dirPath = document.getElementById("mdir").innerHTML;
    var filePaths = [];
    fs.readdir(dirPath, function(err, dir) {
        for (var i = 0, l = dir.length; i < l; i++) {
            var filePath = dir[i];
            filePaths.push(dirPath + "/" + filePath);
        }
        console.log(filePaths);
        console.log(filePaths.length);
    });
}

array.length is zero, but the array has elements in it

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