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How to search for "x" or "any number" in a string?

I ran into a bit of a block when wanting to make a dice rolling script.
The idea is that you can input any amount of arguments to roll any amount of dice, 2d6 + 1d4 + 5d2 - [...].
How can I make a script that finds all the xdy's and puts them into an array? It must be based on xdy instead of, for example, spaces for cases like 1d4+1d6-1d8.

You can extract the pairs of numbers with matchAll, like this:
let s = "1d4+1d6-1d8";
let x = Array.from(s.matchAll(/(-?\d+)d(\d+)/g));
console.log(x);
This gives you an array of triplets. Each triplet has the xdy pattern that was matched, the value of x and the value of y (as strings).
If you need the x and y values as numbers instead of strings, then add a mapper callback to the Array.from call:
let s = "1d4+1d6-1d8";
let x = Array.from(s.matchAll(/(-?\d+)d(\d+)/g), ([s, x, y]) => [+x, +y]);
console.log(x);

Start with creating a function that takes a string (like "2d6") and returns a number
function rollDice(dicePattern) {
var num = ... //use regex or indexOf ("d");
var sides = ... //regex or substring(indexOf("d) + 1);
var sum = 0;
for (let i = 0; i <num;i++) {
//generate a random, multiply by sides, get the int value, and add it to sum
}
return sum;
}
Then split the string and call this on all your dice roll patterns

As stated in the comments, regex is your friend. And regexr is super handy! https://regexr.com/4mvj6
\d will match a digit character, so \dd\d will match any number + 'd' + number combinations in your string.
// '2d6 + 1d4 + 5d2' => ['2d6', '1d4', '5d2']
function parseRolls(rollsString)
{
if (rollsString)
{
return rollsString.match(/\dd\d/g);
}
return [];
}

I'm not completely sure what the exact format you're expecting is, but this returns an array of objects with properties x and y given a string of xdys delimited by plusses or minuses.
function parseDiceRoll(rollString) {
let rollBlocks = rollString.split(/[\+\-]/);
return rollBlocks.map(roll => {
let [x, y] = roll.split("d");
return {x, y};
});
}
console.log(parseDiceRoll("3d6+4d8-2d12"));

Why isn't this code working to find the sum of multiples of 3 and 5 below the given number?

I've started with some problems on HackerRank, and am stuck with one of the Project Euler problems available there.
The problem statement says: Find the sum of all the multiples of 3 or 5 below N
I've calculated the sum by finding sum of multiple of 3 + sum of multiples of 5 - sum of multiples of 15 below the number n
function something(n) {
n = n-1;
let a = Math.trunc(n / 3);
let b = Math.trunc(n / 5);
let c = Math.trunc(n / 15);
return (3 * a * (a + 1) + 5 * b * (b + 1) - 15 * c * (c + 1)) / 2;
}
console.log(something(1000)); //change 1000 to any number
With the values of num I've tried, it seems to work perfectly, but with two out of five test cases there, it returns a wrong answer (I can't access the test cases).
My question is what is the problem with my code? as the logic seems to be correct to me at least.
Edit: Link to problem page

Some of the numbers in the input are probably larger than what javascript can handle by default. As stated in the discussion on the hackkerrank-site, you will need an extra library (like: bignumber.js) for that.
The following info and code was posted by a user named john_manuel_men1 on the discussion, where several other people had the same or similar problems like yours
This is how I figured it out in javascript. BigNumber.js seems to store the results as strings. Using the .toNumber() method shifted the result for some reason, so I used .toString() instead.
function main() {
var BigNumber = require('bignumber.js');
var t = new BigNumber(readLine()).toNumber();
var n;
for(var a0 = 0; a0 < t; a0++){
n = new BigNumber(readLine());
answer();
}
function answer() {
const a = n.minus(1).dividedBy(3).floor();
const b = n.minus(1).dividedBy(5).floor();
const c = n.minus(1).dividedBy(15).floor();
const sumThree = a.times(3).times(a.plus(1)).dividedBy(2);
const sumFive = b.times(5).times(b.plus(1)).dividedBy(2);
const sumFifteen = c.times(15).times(c.plus(1)).dividedBy(2);
const sumOfAll = sumThree.plus(sumFive).minus(sumFifteen);
console.log(sumOfAll.toString());
}
}

Using arrays to do basic calculations with negative exponent

I'm trying to write a function which outputs the correct result when multiplying a number by a negative power of ten using arrays and split() method. For example the following expressions get the right result: 1x10^-2 = 0.01 1x10^-4 = 0.0001.
Problem comes when the number's length is superior to the exponent value (note that my code treats num as a string to split it in an array as shown in code bellow :
//var num is treated as a string to be splited inside get_results() function
//exponent is a number
//Try different values for exponent and different lengths for num to reproduce the problem
//for example var num = 1234 and var exponent = 2 will output 1.234 instead of 12.34
var num = '1';
var sign = '-';
var exponent = 2;
var op = 'x10^'+sign+exponent;
var re = get_result(num);
console.log(num+op +' = '+ re);
function get_result(thisNum) {
if (sign == '-') {
var arr = [];
var splitNum = thisNum.split('');
for (var i = 0; i <= exponent-splitNum.length; i++) {
arr.push('0');
}
for (var j = 0; j < splitNum.length; j++) {
arr.push(splitNum[j]);
}
if (exponent > 0) {
arr.splice(1, 0, '.');
}
arr.join('');
}
return arr.join('');
}
Demo here : https://jsfiddle.net/Hal_9100/c7nobmnj/
I tried different approaches to get the right results with different num lengths and exponent values, but nothing I came with worked and I came to the point where I can't think of anything else.
You can see my latest try here : https://jsfiddle.net/Hal_9100/vq1hrru5/
Any idea how I could solve this problem ?
PS: I know most of the rounding errors due to javascript floating point conversion are pretty harmless and can be fixed using toFixed(n) or by using specialized third-party librairies, but my only goal here is to get better at writing pure javascript functions.

I am not sure if you want to keep going with the array approach to a solution, but it seems like this could be solved with using the Math.pow() method that already exists.
function computeExponentExpression ( test ) {
var base;
var multiplier;
var exponent;
test.replace(/^(\d+)(x)(\d+)([^])([-]?\d+)$/, function() {
base = parseInt(arguments[1], 10);
multiplier = parseInt(arguments[3], 10);
exponent = parseInt(arguments[5], 10);
return '';
} );
console.log( base * Math.pow(multiplier, exponent));
}
computeExponentExpression('1x10^-4');
computeExponentExpression('1x10^2');
computeExponentExpression('4x5^3');

The problem is where you push the decimal point .
instead of
arr.splice(1, 0, '.');
try this:
arr.splice(-exponent, 0, '.');
See fiddle: https://jsfiddle.net/free_soul/c7nobmnj/1/

Create a float from two int numbers in JavaScript

How can I construct a float value from two whole values?
var amountBeforeComma = 5;
var amountAfterComma = 234;
var amount = ?? //amount == 5.234

There's the math way, using logarithms:
var amountBeforeComma = 5;
var amountAfterComma = 234;
var amount = amountBeforeComma +
amountAfterComma * Math.pow(10, -(Math.floor(Math.log10(amountAfterComma)) + 1));
console.log(amount);
Math.log10(amountAfterComma) gives us the common logarithm of amountAfterComma, then Math.floor(...) on that gives us the characteristic of it (2 in your example), which is (as the linked Wikipedia page puts it) "how many places the decimal point must be moved so that it is just to the right of the first significant digit". Then we add one to that and make it a negative (e.g., -3 in your example) and raise raise 10 to that power to get a value to multiply it by (0.001 in your example) to put it where it should go. Add the amountBeforeComma and we're done.
Or the string then parse way:
var amountBeforeComma = 5;
var amountAfterComma = 234;
var amount = parseFloat(amountBeforeComma + "." + amountAfterComma);
console.log(amount);
(Or use +(amountBeforeComma + "." + amountAfterComma) to convert with implicit coercion rather than explicit parsing.)

Since no one mentioned... There's the JavaScript way:
var num = +(amountBeforeComma + "." + amountAfterComma);

You can make it by casting numbers to strings and then parsing it as float.
var amount = parseFloat(amountBeforeComma + '.' + amountAfterComma);

How to sum 2 numbers witout rounding

I have 2 numbers
a = 1548764548675465486;
b = 4535154875433545787;
when I sum these number they are rounded to
a => 1548764548675465500
b => 4535154875433545700
and a + b returns 6083919424109011000 while it should return 6083919424109011273
is there a javascript solution to solve this problem witout the use of a library ?

To work around the precision limitations associated with JavaScript's numbers, you will need to use a BigInteger library like the popular one offered here: http://silentmatt.com/biginteger/
Usage:
var a = BigInteger("1548764548675465486");
var b = BigInteger("4535154875433545787");
var c = a.add(b);
alert(a.toString() + ' + ' + b.toString() + ' = ' + c.toString());
// Alerts "1548764548675465486 + 4535154875433545787 = 6083919424109011273"
Demo: http://jsfiddle.net/69AEg/1/

There are no integers in Javascript, all numbers are double precision floating point.
That gives you a precision of around 15-16 digits, which is what you are seeing.
as per this question
and potential solution i.e. use a library
Personally, I would not use javascript, never been great at numbers. Just try typing 0.1 + 0.2 into any browsers console window. Result is 0.30000000000000004.
Send the calculation to your server side language (as a string) and do the work there, you should have a better outcome.
Technical article on the nuances of floating point numbers here, if you interested

Well, here is a solution I found witout the use of any external library, all I need to do is to define a class that had a property value wich should be a string, and define the function plus
function LongNumber()
{
// it takes the argument and remove first zeros
this.value = arguments[0].toString();
while(this.value[0]==="0")
this.value = this.value.substr(1);
// this function adds the numbers as string to another string and returns result as LongNumber
this.plus = function (Num)
{
var num1 = pad(Num.value.length, this.value);
var num2 = pad(this.value.length, Num.value);
var numIndex = num1.length;
var rest = 0;
var resultString = "";
while (numIndex)
{
var number1 = parseInt(num1[(numIndex)-1]);
var number2 = parseInt(num2[(numIndex--)-1]);
var addition = (number1+number2+rest)%10;
rest = parseInt((number1+number2+rest)/10);
resultString = addition.toString() + resultString;
}
return new LongNumber((rest?rest.toString():"") + resultString);
}
function pad(width, string)
{
return (width <= string.length) ? string : pad(width, '0' + string)
}
}
All i need to do now is to declare 2 LongNombers and use the function plus
var Number1 = new LongNumber("1548764548675465486");
var Number2 = new LongNumber("4535154875433545787");
var Result = Number1.plus(Number2);
Result.value // returns "6083919424109011273"