Change array through a function parameter

Question

I need to change array to a new array created inside a function.

function changeArr(a)
{
 a=["hello"]; // don't work
 //a.push("hello"); //works, but the real array pretty big (dont want copy)
}
var arr=[];
changeArr(arr);
console.log(arr); // should echo ["hello"]

• Alexander: I suggest you read about call by object sharing

Answer 1

It seems like all you really want to do is clear the array that is passed into the function, before appending to it:

function changeArr(a)
{ 
 a.length = 0; // clear the array here if you don't care about the prev. contents
 a.push("hello"); // this adds to the array that was passed in
}

• Jakob Jingleheimer: Since the array is being truncated, you can take advantage of a performance optimisation by setting the first element of the array to "hello" instead of calling a method to do it: a.length = 0; a[0] = "hello";.

Answer 2

Inside the function changeArr(), a is only a local variable referencing the array you passed as an argument when calling this function. a=["hello"] makes this local variable reference a newly created and different array. This changes does not affect the original array passed in. What you want to do is likely what Miky Dinescu suggested: use the local variable to modify the original array but don't assign/attach anything new to it.

Answer 3

If you are trying to completely replace the array, you can use a return value.

function changeArr()
{
 return ["hello"];
}
arr = changeArr();

If there is a reason you aren't doing this, explain and I'll adjust my answer.

Answer 4

You can use the splice method as such:

a.splice(0,a.length,"hello")