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Javascript Pass by Reference

2022-01-18

A bit of hackiness that I ended up using was pass by reference in javascript. Objects in javascript are all passed by reference so when a function gets an object, it will modify the object in place. This means you can move a primitive like a counter into an object and then use that object in a function somewhere else to continue a loop!

I ended up using this logic in my markdown parser. I wrote a function that returns a string and called that function in a few different places. It looped through characters much like my main loop that handled files. This meant that I need a single global counter variable but I don't like using globals especially since I was also using recursion.

Passing a counter variable to a function in javascript results in copying the variable so it won't be changed in the calling function. But if we embed that counter inside an object, voila! We have a pass by reference and we can make modifications to the counter in place!

function incrementCounter(i) {
    for (let k=0;k<5; k++) {
        i++;
    }
}
let i = 0;
console.log(i);
incrementCounter(i);
console.log(i);

This will output 2 0s. This is because i is set to 0 and then the increment only happens on a copy that never gets returned and so i will be 0 even after the incrementCounter function.

function incrementCounter(i) {
    for (let k=0;k<5; k++) {
        i.c++;
    }
}
let i = { c: 0 };
console.log(i.c);
incrementCounter(i);
console.log(i.c);

This will print 0 and 5. I don't like this code because it is quite magical and you wouldn't expect this behavior and there's nothing really in the code that signifies what's about to happen. I would love to be able to add sigil like & so that its obvious when we are doing variable manipulation on a shared object but I did end up using this logic instead of returning multiple values in my function.

I do think doing pass by reference was a good thing here because it simplifies a few things but it does make everything more brittle. I think the real solution might've been to use a class based structure to track variables instead of going the route of writing functions to do everything. I can definitely see how classes would get rid of a lot of the junk code that I have to pass many things around. Maybe at some point I'll take a stab at rewriting my markdown parser.