Memory Management

How Javascript Code Affects memory

Current Memory Available

On a NodeMCU, a program that only has the following line: setTimeout("print(process.memory());", 1); Yields this memory profile: { "free": 992, "usage": 31, "total": 1023, "history": 11 }

According to Espruino Internals, each memory unit is 12 bytes, so we start with something like 12k of memory.

Variables

Setting a global var to a zero sized string:

m='';
setTimeout("print(process.memory());", 1);

{ "free": 991, "usage": 32, "total": 1023, "history": 14 }

The data is stored as a linked list in memory, so the variable itself is one block (>4 char var names take more 12 byte blocks).

Hint: Keep your variable names to a max of 4 characters.

It is not clear where the language elements = and ; are stored. (needs an answer)

Penalty for storing text to a variable.

m='012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789';
print(process.memory());

{ "free": 966, "usage": 57, "total": 1023, "history": 37 }

This is 240 char string takes 360 bytes to store. That is a 50% penalty. According to Espruino Performance It should take Each 12 Bytes should take 16 Bytes

Hint: Minimize the length of variables holding strings.

Literals Apparently Don't Take Any Memory

t='';
if (t==='');
setTimeout("print(process.memory());", 1);

{ "free": 990, "usage": 33, "total": 1023, "history": 20 }

now lets see if t is equal to a 240 char literal:

t='';
if (t==='012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789');
setTimeout("print(process.memory());", 1);

{ "free": 990, "usage": 33, "total": 1023, "history": 20 }

Hint: Literal should be used instead of variables when possible.

12 Bytes per Variable vs 16 Bytes per Variable

If you create more than 256 variables, then Espruino needs 16 bytes per variable.

Regular Javascript arrays Take 2 blocks per value element. There are non-Javascript typed arrays that are more efficient if you data fits the pattern (all Integers, etc.). A small Integer can type one Block per element. See Espruino Performance.

Javascript Objects can be even less memory efficient. Assuming 12 Byte blocks, If your Object index is > 8 bytes, it takes two or more blocks just for the index.

You can see how much memory an element takes using E.getSizeOf(...):

Hint: Carefully engineer JSON objects or replace them with typed Arrays.

Avoid Long Commands

String.fromCharCode() If you have something like this scattered throughout your program, make a short named function.

When you create functions, give them short names. This is mitigated (actually rendered unnecessary if you turn on Closure [Simple Optimizations] in the Minification part of the IDE).