Javascript confusing order of execution

October 6, 2022

I was playing around with reverse() to see how it works and came across this interaction that I can’t understand at all. Here is the initial code:

let muhArray = [1,2,3,4,5];

let reversedArray = muhArray.reverse();

console.log(`muhArray: ${muhArray}`);

console.log(`reversedArray: ${reversedArray}`);

This returns:

muhArray: 5,4,3,2,1 (Original array changed as well)

reversedArray: 5,4,3,2,1

I figured that since reverse() reverses arrays in-place, even if it is an initilization, it changes the original muhArray anyways. So I created a copy(muhArrayCopy) to preserve the original array.

let muhArray = [1,2,3,4,5];

let muhArrayCopy = muhArray;

console.log(`muhArrayCopy: ${muhArrayCopy}`);

let reversedArray = muhArray.reverse();

console.log(`reversedArray: ${reversedArray}`);

This returns:

muhArrayCopy: 1,2,3,4,5

reversedArray: 5,4,3,2,1

Up until this point everything seems fine, but changing the order made me realise muhArray.reverse() affects already declared and initialized muhArrayCopy as well.

let muhArray = [1,2,3,4,5];

let muhArrayCopy = muhArray;

let reversedArray = muhArray.reverse();

console.log(`muhArrayCopy: ${muhArrayCopy}`);

console.log(`reversedArray: ${reversedArray}`);

Such order returns:

muhArrayCopy: 5,4,3,2,1

reversedArray: 5,4,3,2,1

Is this maybe a code execution order thing? I have read about hoisting but that does not seem to explain it, as far as I can understand. I am all new to programming so I have little to no knowledge about it.

Thanks in advance.

>Solution :

js is call by reference, so

let muhArrayCopy = muhArray;

does not create a copy, it puts the reference of variable in the other one, so both are affected when something happens to the other.

let muhArray = [1,2,3,4,5];

let muhArrayCopy = [...muhArray];

let reversedArray = muhArray.reverse();

console.log(`muhArrayCopy: ${muhArrayCopy}`);

console.log(`reversedArray: ${reversedArray}`);

this should solve it. here [...muhArray] we are creating a copy of the array.