array.length is zero, but the array has elements i

问题:

This question already has answers here:

Is Chrome's JavaScript console lazy about evaluating arrays? (6 answers)

Closed 2 years ago.

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?

回答1:

readdir is asynchronous. It won't get the result right away. You should log the filePaths inside the callback. The only reason why the console show the value is because the console evaluate 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 happen is that the console keeps a reference to the array, so when the user unfold the array it then show what its current value is. 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 already be done reading and the console will print the current value of the array (after it been filled).

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

Try and run this code and see what happen:

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 array[]. Now because arrays could have tons of data, the console won't show that data untill the user unfold the array. So what the console does is keep a reference to the array untill the user press unfold arrow. If you unfold the array before 5 seconds you see that the array is empty (not filled yet). If you wait untill the 5 seconds pass then unfold it, then you'll see that it's filled.

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.

回答2:

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);
    });
}