Variable value constantly updating rather than staying the same as originally defined value-CodePudding

I am trying to get a create a variable oldPlayerStats to get the current value of G.playerStats, so that later when G.playerStats gets updated, I can subtract the oldPlayerStats from the new value of G.playerStats to get the difference.

However for some reason, oldPlayerStats updates to always match G.playerStats.

Relevant code below:

const oldPlayerStats = G.playerStats;
console.log(oldPlayerStats[0].wood); //Is 10 as it should be

//This function affects the value of `G.playerStats`. It does not do anything to oldPlayerStats
cardFunction.function(G, ctx, ctx.currentPlayer, G.opponent, G.attackMultiplier);
    
console.log(oldPlayerStats[0].wood); //Should be 10, but instead updates to match the new value of `G.playerStats`

CodePudding user response：

Right now oldPlayerStats is just referencing the original object in memory, and that is why it is changing.

One option is to clone the object using structured clone to create a deep clone.

https://developer.mozilla.org/en-US/docs/Web/API/structuredClone

const oldPlayerState = structuredClone(G.playerState)

CodePudding user response：

In Javascript, when you assign a variable to another variable, you are doing so by reference. So instead of saying "i want oldPlayerStats to equal the value of G.playerStats", it's saying oldPlayerStats points to the memory address where G.playerStats is stored. Therefore when you update one, it will update the other.

you can use slice to copy the values from an array into a new array.

oldPlayerStats = G.playerStats.slice()

CodePudding user response：

I`m far from being an expert in JS, but in most languages, assigning complex data structure to another variable, uses reference - the data is not copied, but the new variable, points to the same place in memory. Therefore, if you change some of the original datastructure's elements, but not the adress in memory, the new variable will still show the updated value of the original one. There are several methods to avoid it. In case of JavaScript, this should help: https://www.freecodecamp.org/news/how-to-clone-an-array-in-javascript-1d3183468f6a/

CodePudding user response：

Although the provided answers all made sense and there was plenty of other documentation supporting those answers as possible solutions, neither of them worked.

What did end up working for me was:

const oldPlayerStats = JSON.parse(JSON.stringify(G.playerStats));

This solution came from the following question I found: Uncaught DOMException: Failed to execute 'postMessage' on 'Window': An object could not be cloned