Array-of-struct emulation using TypedArray and some utility functions -- fast, contiguously allocated arrays unlike regular JS arrays of objects.

aos.js

const systemWordSize = 8; //bytes
const strideLength = 1; //bytes
//...these functions can be optimised to take advantage of 4 or 8 byte reads / writes, depending on system word size.

//Utility functions

function padAndCalcMemberOffsets(type)
{
    let structSize = 0;
    for (let memberName in type)
    {
		let member = type[memberName];
		console.log(memberName, member);
		member.offset = structSize;
        structSize += member.size;
    }
	
	let remainder = structSize % systemWordSize;
	if (remainder != 0)
	{
		let padSize = systemWordSize - remainder;
		type._padding = {size: padSize, offset: structSize};
	
		structSize += padSize;
	}
	
    return structSize;
}

function read(array, index, type, memberName)
{
	let bytesLength = type[memberName].size;
	let ibase = index * type._size;
	let i = ibase + type[memberName].offset;
	let iStart = i;
	let value = 0;
	
	//read out sequentially into a single multibyte value.
	for (let b = 0; b < bytesLength; b += strideLength)
	{
		let bitsToShiftBy = b * 8; //bits per byte
		value |= array[i] << bitsToShiftBy;
		i += strideLength; //1 byte at a time
	}
	
	console.log("read ", type._name, "at ["+ibase+"],",  memberName, "at ["+iStart+"] (length="+bytesLength+"):", value);
	
	return value;
}

//write a multibyte field.
function write(array, index, type, memberName, value)
{
	let bytesLength = type[memberName].size;
	let ibase = index * type._size;
	let i = ibase + type[memberName].offset;
	let iStart = i;
	let valueNew = 0;
	
	//read out sequentially into a single multibyte value.
	for (let b = 0; b < bytesLength; b += strideLength)
	{
		let bitsToShiftBy = b * 8; //bits per byte
		let valueSegment = (value >> bitsToShiftBy) & 255;
		
		array[i] = valueSegment;
		i += strideLength; //1 byte at a time
	}
	
	console.log("write", type._name, "at ["+ibase+"],",  memberName, "at ["+iStart+"] (length="+bytesLength+"):", value);
}

function prepareStructTypes()
{
	for (let structTypeName in structTypes)
	{
		let structType = structTypes[structTypeName];
		
		structType._size = padAndCalcMemberOffsets(structType);
		structType._name = structTypeName; //optional, only used for console.log
		console.log(structTypeName, structType);
	}	
}

//Set up structs

let structTypes = {
	
	Boxer:
	{
		id:   {size: 2, offset: 0},
		hits: {size: 3, offset: 0},
		time: {size: 4, offset: 0},
		//total size of this struct would be 1+4+2=7 bytes, which does not align with
		//typical 4 or 8 byte word boundaries, which make access (very) inefficient. So pad it.
	},
	
	Javelineer:
	{
		a: {size: 1, offset: 0},
		b: {size: 3, offset: 0},
	},

}

prepareStructTypes(structTypes); //pad them, figure out member offsets, and get struct total size.

//Test

const Boxer = structTypes.Boxer;

const arrayOfBoxer = new Uint8Array(8 * Boxer._size);
console.log("Interleaved Boxer array length:", arrayOfBoxer.length, "size per element", Boxer._size);



let i = 3;
let id = 888;
let time = 23983291;

//write
write(arrayOfBoxer, i, Boxer, "id", id); // i * Boxer._size + Boxer.id.offset,
write(arrayOfBoxer, i, Boxer, "time", time); // i * Boxer._size + Boxer.id.offset,
 
console.log(arrayOfBoxer);

//read
read(arrayOfBoxer, i, Boxer, "id");
read(arrayOfBoxer, i, Boxer, "time");

Outputs:

Boxer {
    "id": {
        "size": 2,
        "offset": 0
    },
    "hits": {
        "size": 3,
        "offset": 2
    },
    "time": {
        "size": 4,
        "offset": 5
    },
    "_padding": {
        "size": 7,
        "offset": 9
    },
    "_size": 16,
    "_name": "Boxer"
}

And:

aos.js:70 write Boxer at [48], time at [53] (length=4): 23983291
aos.js:125 Uint8Array(128) [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 120, 3, 0, 0, 0, 187, 244, 109, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, …]
aos.js:46 read Boxer at [48], id at [48] (length=2): 888
aos.js:46 read Boxer at [48], time at [53] (length=4): 23983291