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How to convert a value too large for integers in JavaScript?

I am fairly new to this and please let me know if I am wrong in any point.
I am trying to convert WEI into Eth, without using the Web3 library.
const weiValue = 100000;
const ethValue = ethers.utils.formatEther(weiValue);<--- answer is correct at 0.0000000000001
My code requires me to change 40000000000000000 wei to eth.
and here is what I have:
const weiValue = 40000000000000000;
const tokenRateString = tokenRate.toString();
const ethValue = ethers.utils.formatEther();
I believe I need to convert the Wei into a string as it too large to be represented accurately as integers in JS
I am confused on what is the intermediate step here

To who ever comes here later.
Yes, a big number such like this : 40000000000000000 need to be a string.
What you need to do is this: 40000000000000000
const WeiToEth = (tokenRate)=> {
const weiValue = BigNumber.from("40000000000000000");
const ethValue = ethers.utils.formatEther(weiValue);
console.log(ethValue) }
It converts the number into a string
Here is the documentation : https://docs.ethers.io/v5/api/utils/bignumber/

why does digest and digest('hex') result in different outputs?

I have 2 piece of codes.
1ST ONE
const hash1 = (data) => createHash('sha256').update(data).digest('hex');
var a1 = hash1("A");
var b1 = hash1("B");
console.log(hash1(a1+b1));
2ND ONE
const hash2 = (data) => createHash('sha256').update(data).digest();
var a2 = hash2("A");
var b2 = hash2("B");
console.log(hash2(Buffer.concat([a2,b2])).toString('hex'));
Why do they print the different results ?
digest('hex') and digest() are the same , but in different kind of format, but still the same. So, why do I get different results in console ? is it the + operator when I sum up the hexes versus when i sum up buffers ? why ?

The default encoding for hash.digest([encoding]) is utf-8. utf-8 is a variable-length encoding system. It will only use as many bytes as necessary to represent each character (anywhere between 1-4 bytes).
However, when you specify hex as the encoding, each character is stored as exactly 2 hexadecimal characters.
When you call hash.toString('hex') on a utf-8 encoded hash, the resulting hex representation is equivalent to hashing with hex encoding in the first place (as in hash.digest('hex')).
So, even though the hex representation is the same in each case, the actual data is different. i.e.:
hash.digest() != hash.digest('hex'), but
hash.digest().toString('hex') == hash.digest('hex').

digest('hex') and digest() technically different
Please try this code
var crypto = require('crypto');
const hash1 = (data) => crypto.createHash('sha256').update(data).digest('hex');
var a1 = hash1("A");
var b1 = hash1("B");
//console.log(a1)
//console.log(b1)
console.log(hash1(a1+b1));
const hash2 = (data) => crypto.createHash('sha256').update(data).digest();
var a2 = hash2("A");
var b2 = hash2("B");
//console.log(a2.toString("hex"))
//console.log(b2.toString("hex"))
console.log(hash2(a2.toString("hex") + b2.toString("hex") ).toString("hex"));
In the first you are appending two hex string and passing to hash1
In the second you are appending two non hex string and passing to hash2
Run Nodejs online

JavaScript: reading 3 bytes Buffer as an integer

Let's say I have a hex data stream, which I want to divide into 3-bytes blocks which I need to read as an integer.
For example: given a hex string 01be638119704d4b9a I need to read the first three bytes 01be63 and read it as integer 114275. This is what I got:
var sample = '01be638119704d4b9a';
var buffer = new Buffer(sample, 'hex');
var bufferChunk = buffer.slice(0, 3);
var decimal = bufferChunk.readUInt32BE(0);
The readUInt32BE works perfectly for 4-bytes data, but here I obviously get:
RangeError: index out of range
at checkOffset (buffer.js:494:11)
at Buffer.readUInt32BE (buffer.js:568:5)
How do I read 3-bytes as integer correctly?

If you are using node.js v0.12+ or io.js, there is buffer.readUIntBE() which allows a variable number of bytes:
var decimal = buffer.readUIntBE(0, 3);
(Note that it's readUIntBE for Big Endian and readUIntLE for Little Endian).
Otherwise if you're on an older version of node, you will have to do it manually (check bounds first of course):
var decimal = (buffer[0] << 16) + (buffer[1] << 8) + buffer[2];

I'm using this, if someone knows something wrong with it, please advise;
const integer = parseInt(buffer.toString("hex"), 16)

you should convert three byte to four byte.
function three(var sample){
var buffer = new Buffer(sample, 'hex');
var buf = new Buffer(1);
buf[0] = 0x0;
return Buffer.concat([buf, buffer.slice(0, 3)]).readUInt32BE();
}
You can try this function.

Convert 64 bit Steam ID to 32 bit account ID

How do I convert a 64 bit steam ID to a 32 bit account ID? Steam says to take the first 32 bits of the number, but how do you do this in Node?
Do I need to use BigNumber to store the 64 bit int?

To convert a 64 bit Steam ID to a 32 bit Account ID, you can just subtract 76561197960265728 from the 64 bit id.
This requires bigNumber in node:
bignumber = require("bignumber.js");
console.log(bignumber('76561197991791363').minus('76561197960265728'))

I had the same issue but didn't want to use any library like bignumber.js as my project was quite small and will be used in a web browser. In the end I came up with this elegant solution:
function steamID64toSteamID32 (steamID64) {
return Number(steamID64.substr(-16,16)) - 6561197960265728
}
How it works:
To get the lower 32 bits we need to convert the SteamID64 string to a number, but because JavaScript has a precision limit of 57 bits the SteamID64 will be erroneously rounded. The workaround is to truncate the leftmost digits to get a 16 digit number, which uses at most 54 bits and will therefore retain its precision in Javascript. This is acceptable because the leftmost digits comes from the higher 32 bits which will be zeroed anyway, so nothing of value will be lost.
To zero the remaining higher bits we subtract the decimal number they're representing. If we assume that every SteamID64 we're converting to be in the public universe this decimal number will be constant and can be computed like this:
1. 0b00000001000100000000000000000001 0b00000000000000000000000000000000 = 76561197960265728
2. Number('76561197960265728'.substr(-16,16)) = 6561197960265728

Here's what I came up with. I started learning JavaScript yesterday (coming from a C++ background, not very accustomed to working without types), so correct me if I did something derpy with the language. I tested it with my own steam ID and it seems to work.
// NOTE: Functions can take in a steamID in its packed 64-bit form
// (community ID starting with 765), its modern form with or without
// either or both brackets, and its legacy form. SteamID's that
// contain letters (e.g. STEAM_0... or [U:1...) are not case-sensitive.
// Dependencies: BigInteger library, available from http://silentmatt.com/biginteger/
// Global variable used by all conversion functions
var STEAM_BASELINE = '76561197960265728';
// IN: String containing a steamID in any of the 3 formats
// OUT: String containing the steamID as a community ID (64-bit packed ID)
function ConvertToPacked(inputID)
{
var output = "unknown";
// From packed
if(inputID.match(/^765/) && inputID.length > 15)
{
output = inputID;
}
// From modern
else if(inputID.match(/^\[U:1:/i) || inputID.match(/^U:1:/i))
{
var numericPortion = inputID.replace(/^\[U:1:|^U:1:/i,'').replace(/\]/,'');
output = BigInteger.add(numericPortion, STEAM_BASELINE).toString();
}
// From legacy
else if(inputID.match(/^STEAM_0:[0-1]:/i))
{
var splitID = inputID.split(":");
var product = BigInteger.multiply(splitID[2],2);
var sum = BigInteger.add(product, STEAM_BASELINE);
output = BigInteger.add(sum, splitID[1]).toString();
}
return output;
}
// IN: String containing a steamID in any of the 3 formats
// OUT: String containing the steamID in the modern format (e.g. [U:1:123456])
function ConvertToModern(inputID)
{
var output = "unknown";
// From packed
if(inputID.match(/^765/) && inputID.length > 15)
{
output = "[U:1:" + BigInteger.subtract(inputID, STEAM_BASELINE).toString() + "]";
}
// From modern
else if(inputID.match(/^\[U:1:/i) || inputID.match(/^U:1:/i))
{
var numericPortion = inputID.replace(/^\[U:1:|^U:1:/i,'').replace(/\]/,'');
output = "[U:1:" + numericPortion + "]";
}
// From legacy
else if(inputID.match(/^STEAM_0:[0-1]:/i))
{
var splitID = inputID.split(":");
var numeric = BigInteger.add(BigInteger.multiply(splitID[2],2), splitID[1]);
output = "[U:1:" + numeric.toString() + "]";
}
return output;
}
// IN: String containing a steamID in any of the 3 formats
// OUT: String containing the steamID in the legacy format (e.g. STEAM_0:0:123456)
function ConvertToLegacy(inputID)
{
var output = "unknown"
// From packed
if(inputID.match(/^765/) && inputID.length > 15)
{
var z = BigInteger.divide(BigInteger.subtract(inputID, STEAM_BASELINE), 2);
var y = BigInteger.remainder(inputID, 2);
output = 'STEAM_0:' + y.toString() + ':' + z.toString();
}
// From modern
else if(inputID.match(/^\[U:1:/i) || inputID.match(/^U:1:/i))
{
var numericPortion = inputID.replace(/^\[U:1:|^U:1:/i,'').replace(/\]/,'');
var z = BigInteger.divide(numericPortion, 2);
var y = BigInteger.remainder(numericPortion, 2);
output = 'STEAM_0:' + y.toString() + ':' + z.toString();
}
// From legacy
else if(inputID.match(/^STEAM_0:[0-1]:/i))
{
output = inputID.toUpperCase();
}
return output;
}

Best way to generate unique ids client-side (with Javascript)

I need to generate unique ids in the browser. Currently, I'm using this:
Math.floor(Math.random() * 10000000000000001)
I'd like to use the current UNIX time ((new Date).getTime()), but I'm worried that if two clients generate ids at the exact same time, they wouldn't be unique.
Can I use the current UNIX time (I'd like to because that way ids would store more information)? If not, what's the best way to do this (maybe UNIX time + 2 random digits?)

you can create a GUID using the following links:
http://softwareas.com/guid0-a-javascript-guid-generator
Create GUID / UUID in JavaScript?
This will maximise your chance of "uniqueness."
Alternatively, if it is a secure page, you can concatenate the date/time with the username to prevent multiple simultaneous generated values.

https://github.com/uuidjs/uuid provides RFC compliant UUIDs based on either timestamp or random #'s. Single-file with no dependencies, supports timestamp or random #-based UUIDs, uses native APIs for crypto-quality random numbers if available, plus other goodies.

In modern browser you can use crypto:
var array = new Uint32Array(1);
window.crypto.getRandomValues(array);
console.log(array);

var c = 1;
function cuniq() {
var d = new Date(),
m = d.getMilliseconds() + "",
u = ++d + m + (++c === 10000 ? (c = 1) : c);
return u;
}

Here is my javascript code to generate guid. It does quick hex mapping and very efficient:
AuthenticationContext.prototype._guid = function () {
// RFC4122: The version 4 UUID is meant for generating UUIDs from truly-random or
// pseudo-random numbers.
// The algorithm is as follows:
// Set the two most significant bits (bits 6 and 7) of the
// clock_seq_hi_and_reserved to zero and one, respectively.
// Set the four most significant bits (bits 12 through 15) of the
// time_hi_and_version field to the 4-bit version number from
// Section 4.1.3. Version4
// Set all the other bits to randomly (or pseudo-randomly) chosen
// values.
// UUID = time-low "-" time-mid "-"time-high-and-version "-"clock-seq-reserved and low(2hexOctet)"-" node
// time-low = 4hexOctet
// time-mid = 2hexOctet
// time-high-and-version = 2hexOctet
// clock-seq-and-reserved = hexOctet:
// clock-seq-low = hexOctet
// node = 6hexOctet
// Format: xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
// y could be 1000, 1001, 1010, 1011 since most significant two bits needs to be 10
// y values are 8, 9, A, B
var guidHolder = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx';
var hex = '0123456789abcdef';
var r = 0;
var guidResponse = "";
for (var i = 0; i < 36; i++) {
if (guidHolder[i] !== '-' && guidHolder[i] !== '4') {
// each x and y needs to be random
r = Math.random() * 16 | 0;
}
if (guidHolder[i] === 'x') {
guidResponse += hex[r];
} else if (guidHolder[i] === 'y') {
// clock-seq-and-reserved first hex is filtered and remaining hex values are random
r &= 0x3; // bit and with 0011 to set pos 2 to zero ?0??
r |= 0x8; // set pos 3 to 1 as 1???
guidResponse += hex[r];
} else {
guidResponse += guidHolder[i];
}
}
return guidResponse;
};

You can always run a test against existing IDs in the set to accept or reject the generated random number recursively.
for example:
const randomID = function(){
let id = Math.floor(Math.random() * 10000000000000001) + new Date();
if (idObjectArray.contains(id)) {
randomID;
} else {
idObjectArray.push(id);
}
};
This example assumes you would just be pushing the id into a 1D array, but you get the idea. There shouldn't be many collisions given the uniqueness of the random number with the date, so it should be efficient.

There are two ways to achieve this
js const id = Date.now().toString()
While this does not guarantee uniqueness (When you are creating multiple objects within 1ms), this will work on a practical level, since it is usually not long before the objects on the client are sent to a real server.
If you wanted to create multiple records withing 1ms, I suggest using the code below
const { randomBytes } = require("crypto");
// 32 Characters
const id = randomBytes(16).toString("hex");
It works similar to a uuid4 without needing to add an external library (Assuming you have access to NodeJs at some point)