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VistasHow is memory allocation for recursion with arrays in JavaScript
I want to understand, when I send an globalArray as a parameter to a recursive function, and assign the same array (localArray) to the globalArray, how is the memory allocation happening in javascript. I know that each recursion will have it's own callstack, and it will have it's own copy of variables, but since I am passing a globalArray into the function, will a copy of the localArray be always stored as a new array in each callstack?
If I were to achieve the localArray as an incremental copy of the same localArray variable for each execution context, how would I be achieving that?
Please view the image and code snippet
function testFunction(localArray){
globalArray = localArray;
if(itr < 5) {
globalArray.push(1);
itr++;
testFunction(localArray);
}
}
var globalArray = [1,2,3];
var itr = 0;
testFunction(globalArray);
[added later to this question]
But if I were to modify the code a bit where I were to make the localArray as null, the globalArray would still be holding data, if it was a value which was referred.
function testFunction(localArray){
globalArray = localArray;
localArray = null;
if(itr < 5) {
globalArray.push(1);
itr++;
testFunction(globalArray);
}
}
var globalArray = [1,2,3];
var itr = 0;
testFunction(globalArray);
1 Respuestas
Responde la pregunta0
... since I am passing a globalArray into the function, will a copy of the localArray be always stored as a new array in each callstack?
No. As the commenters on your question pointed out, arrays are passed as values by reference, meaning the globalArray variable represents the place in memory where that array exists. If you change a variable pointing to an array (i.e. globalArray = ...), you're only changing that variable. But if you modify an array (e.g. globalArray.push(...)), you're changing its contents for every variable that's looking to that array in memory, regardless of which variable name you use, or where it exists on the stack.
If I were to achieve the localArray as an incremental copy of the same localArray variable for each execution context, how would I be achieving that?
There are a few ways to do something like that.
For example, you could clone the array before passing it to the next level of recursion, but if you ended up recursing very deeply that could use a lot of time and memory creating arrays and copying values that you might never use.
Alternatively, rather than using arrays, you could create a memory structure similar to a linked list node, and each level adds a node to the end of the previous one to pass as a variable. That way it's still possible to traverse all the elements from the call stack when you need to, but you use an amount of memory that's proportionate to the depth of the call stack.
You could also reuse a single global array, which you push onto as the call stack gets deeper, but you pop off the values before you return from each level of recursion: basically maintaining a stateful stack parallel to the call stack.
The best approach will likely depend on the details of your recursive algorithm, and how much data you actually need to maintain at each level. In my experience, the local parameters passed in to each level are usually relatively "flat" summary values, such as the current depth of recursion, whereas the global variables are the ones where you maintain state.