setting hardware configuration bits in hardware using JS

Question

My problem can be reduced to this trivial (or so it seemed) bit of code

var myArray = [1,2];
function addOnetoArrayElement(element){
element +=1;
console.log(element);     // Returns 3
}

addOnetoArrayElement(myArray[1]);
console.log(myArray[1]);  // Returns 2

I find in fascinating that one can do all sorts of operations on arrays like map, reduce, push to an array but no reference at all to doing simple arithmetic operations of elements of an array. Never mind all that stuff about hoisting,scope, passing by value and reference. I'm sure that maybe someday I'll know enough to contribute to that discourse. What must I do to make myArray[1] = 3?

score 0 · Answer 1 · answered Feb 27 '21 at 01:39

You assume that myArray[1] will be passed to the function in a way that it can modify that element in myArray (so-called by reference).

But myArray[1] is just a number, it's a primitive type and is passed by value. It means a copy is passed to the function which has no connection to myArray. As a result modification of element param does not change myArray (see docs).

Primitive parameters (such as a number) are passed to functions by value; the value is passed to the function, but if the function changes the value of the parameter, this change is not reflected globally or in the calling function.

For reference there are just 7 primitive data types in JS: string, number, bigint, boolean, undefined, symbol, and null.

To fix the issue we need to pass whole myArray to function as first parameter (by reference because it's an Array) and index as second. In this case myArray will be passed by reference and the function will be able to modify it.

var myArray = [1,2];
function addOnetoArrayElement(arr, ind){
  arr[ind] += 1;
  console.log(arr[ind]);     // Returns 3
}

addOnetoArrayElement(myArray, 1);
console.log(myArray[1]);  // Returns 3

setting hardware configuration bits in hardware using JS

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