In JavaScript What's the difference between variable having value and passing that value directly

user_8247748 0
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I'm new to development, just need to know in terms of memory does it matter using a value without assigning in to a variable

Example 1:

var sendResponse = {
    "a": 1,
    "b": 2
};
res.send(sendResponse);

OR

res.send({
    "a": 1,
    "b": 2
});

Example 2:

var valueOne = "hi";
var valueTwo = "hithere";

    if (someVariable  == valueOne  || someVariable == valueTwo) {
        //
    }

    OR

    if (someVariable  == "hithere" || someVariable == "hi") {
        //
    }

Example 3:

var concatOne = "hi";
    var concatTwo = "hithere";

    var concatString = concatOne + concatTwo;

    OR

    var concatString = "hi" + "hithere";
node.js memory memory-management variables javascript
2 Answers

Short answer:

It doesn't matter.

Cop-out answer:

What you're asking is basically the JavaScript equivalent of "How much memory does an rvalue take?" in C++, which has a good answer here.

Longer answer:

If you absolutely care about memory usage and performance then you shouldn't be using JavaScript in the first place (yes, modern JS JITs are very fast and are fast-approaching traditional AOT-compiled languages like C++, but there's still a huge memory tradeoff and first-run performance-hit for the JIT).

Secondarily, the kind of optimization you're referring to is a microoptimization - even if there are savings to be made by putting string-literals directly as arguments it would maybe save you maybe a handful of bytes for the entire script execution, compare that to the tens-to-hundreds of megabytes of memory that V8 will happily use (and that's still a fraction of the presumably dozens of gigabytes of RAM that your computer has).

(The technical details below are generalities and are not specific to V8 - which I'm not intimately familiar with, though I do have first-hand experience with the internals of Chakra from my time at Microsoft).

Some things to consider:

  • Most importantly: the JavaScript specification does not dictate how a JS engine is implemented. Someone could build a horribly inefficient engine that uses a gigabyte of internal memory for each variable and still comply with the spec. To give a perfect answer to your question would require a through and in-depth knowledge of V8's internals - and such knowledge would be outdated and obsolete within a few years anyway given how fast the JS ecosystem changes.

  • Most modern JavaScript engines (including V8) generally have 3 modes of operation:

    1. Parsing.
    2. Interpreted mode.
    3. JIT mode.

  • Before a script can be executed it needs to be parsed: and when a script is being parsed any string-literals will be interned and kept in read-only memory (and so the string values consume a fixed, known, quantity of memory), which means that those values will never be destroyed by the GC.

    • Side-note: some systems (like Win32 and Prolog) refer to these as "string atoms" or "atomic strings" instead of "interned strings", with subtly different semantics.
    • (I accept that string-literals in dynamically evaluated scripts (namely in eval()) may be reclaimed - however I don't believe (but I could easily be wrong!) that any JS engines' GC today will free string literals that have no references pointing at them.

  • After a string literal has been interned it's possible for a JS engine's user-script variables to use some internal tagged reference type to succinctly refer to the string without needing to use a full 8-byte internal pointer (similar to, but not exactly nor necessarily a tagged pointer), there is also the possibility for small-string-optimization to be used (where a local copy of the entire string, if it's small enough, is used instead of a pointer or reference to the string - this is fairly commonplace in C++ to avoid the runtime costs of multiple indirection).

  • Don't conflate memory used by a reference or local to the memory used by the value.

    • Memory usage by references/locals is difficult to identify because any fairly decent JIT will perform a lot of operations entirely within CPU registers and other non-memory addresses - as well as predetermining many operations as though they were like C++ constexpr - so if a variable is elided away does it still use any memory? does it use "zero" bytes? that's more a philosophical question, imo.

  • (there's loads more I could write on this subject but it's 7am now and I'm off...)

user_1591450

Yes it does, Because when you declare a variable in JS we are not specifying the size that variable going to hold so it allocates a minimal space for every variable we create , as well as the names we give to every variable will also take some space. So in case of dynamic allocation its okay do that , But the second method in every example is the preferred one.

var concatOne = "hi";
var concatTwo = "hithere";

var concatString = concatOne + concatTwo;

here the "var" keywords does not know which data type your going to use so it will be allocated the most heights data type size like big int , and also the two variable names also holds some space.

But the another method is priffered to be a good practice with optimized memory use as well as shorter code size, which can improve your code working performance the compilation and run time will be considerably lower

  var concatString = "hi" + "hithere";
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