Referencing Arrays in Javascript

Question

I've got what may be a stupid question. In the code below, the function doStuff appears to reassign myArray to an empty array, but when tried it in the console, myArray is still [2,3,4,5].

var myArray = [2, 3, 4, 5];
function doStuff(arr) {
   arr = [];
};

doStuff(myArray);
console.log(myArray)// => [2,3,4,5]

Further, a function that modifies the array seems to work fine. For example:

 function changeSecondIndex(arr){
      arr[2] = 25;
 }

 changeSecondIndex(myArray)
 console.log(myArray) // => [2,3,25,5]

Could someone please help me understand what's going on here? Thanks.

Answer 1

Your code is creating new empty array, instead you can empty existing array with length = 0

var myArray = [2, 3, 4, 5];
function doStuff(arr) {
   arr.length = 0;
};

doStuff(myArray);
console.log(myArray)

Answer 2

Arrays are passed by reference in JavaScript. So, in your doStuff and changeSecondIndex functions, the arr parameter contains a reference to myArray.

In changeSecondIndex, you're using that reference to access and update a property of the array. That works fine, as you can see. But in doStuff, what's actually happening is that you are setting arr to a new array. What that does is remove the fact that arr was a reference to myArray and sets it to a new blank array. That's why myArray is not modified.