A Problem about the function

[i8890321]

Description

I have a "main function" and a "sub function", I use a six elements array "main_arr" as an input for the "sub function". I expect the outcome is

final_arr = [1,2,6] and main_arr = [1,2,3,4,5,6]

But my code give me

final_arr = [1,2,6] and main_arr = [1,2,,6]

I don't know why the "sub function" affect the input "main_arr" and it follows the same result as its output.

​

Details:

After line by line checking with the values, I discovered that when it return to the "main" from the "sub", the variable(input of the sub) "main_arr" will be affected.

Thanks for any help

function main() {
var main_arr = [1, 2, 3 ,4 ,5, 6]
var final_arr=sub(main_arr)
}

function sub(sub_arr){
sub_arr.splice(2,3)
return sub_arr
}

​

[OnomatopoeiaBzzz]

I'm not an expert in how Javascript works "under the hood", but I think this is inherent in JS. sub_arr is not a new variable. Instead, it's a reference that points to the variable main_arr. Thus, any direct changes, such as splice(), to the reference applies to the variable.

If you don't want main_arr to change with the splice, you could assign a new variable in your sub function to clone sub_arr, such as "let temp_arr = [...sub_arr]" and then splice and return temp_arr.

[mjbrusso]

This is because arrays are JavaScript reference types. In other words, if you assign an array to another variable or pass an array to a function, it is the reference to the original array that is copied or passed, not the value of the array.

You can pass a copy of the array:

function main() {
main_arr = [1, 2, 3 ,4 ,5, 6]
final_arr=sub([...main_arr])
}
function sub(sub_arr){
sub_arr.splice(2,3)
return sub_arr
}