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I dont know what these symbols in this code mean. Decimal to binary

function dec2bin (decimal) {
var x = document.getElementById("deci").value;
if ((/[^0-9]/g.test(x)) || x == "") {
alert ("You must enter an integer decimal number!");
document.getElementById("deci").value = "";
document.getElementById("deci").focus();
return false;
}
x = parseInt(x);
var bin = x.toString(2);
var figs = "The binary representation of " + x + " is " + bin + "<br>";
document.getElementById("result").innerHTML = figs;
This is the sample code my teacher gave us. It is a decimal to binary converter. I'm not entirely sure what some of these symbols mean. For example, the if statement. What exactly does
(/[^0-9]/g.test(x)) || x == "")
do? Also what exactly does
x = parseInt(x);
var bin = x.toString(2);
accomplish, I kind of understand but would like futher explanation .

/[^0-9]/g is a regular expression that matches anything that isn't a decimal digit. So /[^0-9]/g.test(x) will be true if x contains any non-digits. x == "" is true if x is an empty string. Combining them, (/[^0-9]/g.test(x)) || x == "") is true if x is empty or contains non-digits. In other words, it's true if the input isn't a sequence of decimal digits. You can learn more about regular expressions at http://www.regular-expressions.info/
x = parseInt(x); calls the Javascript parseInt() function, which converts a string representing an integer into an integer. BTW, you should always provide the second argument to specify the radix, as it may differ between implementations; it should usually be x = parseInt(x, 10); to indicate that you're parsing decimal.
var bin = x.toString(2); calls the Javascript Number.prototype.toString method. This converts a number to a string in the specified output radix. Radix 2 is binary.

Random generator in Javascript with min and max value not working

I am new to JavaScript. I am trying to make an exercise program which generates a random number between a min and a max value. I am facing an issue in the program below. var2 + min is not working correctly. If I replace variable min with the actual value, then it works. What am I doing wrong?
var var1=Math.random()
var min = prompt("Enter Min value:")
var max = prompt("Enter max value:")
alert("min is "+min+" max is "+max)
var var2=var1*(max-min)
var var3=var2+min
var var4=Math.floor(var3)
alert("var1= "+var1+" var2= "+var2+" var3= "+var3+" Var4 "+var4)

Use:
var min = parseInt(prompt("Enter Min value:"), 10);
var max = parseInt(prompt("Enter max value:"), 10);
The problem is that these variables contain strings, so the expressions containing + are performing string concatenation rather than number addition.
And while you're learning, get in the habit of ending statements with ;. Javascript is lax about requiring this, but you should be explicit about it -- the rules for when semicolon can be omitted are a bit arcane.

var1, var2, var3, etc. are not good variable names. Don't use them.
Your code isn't working because prompt returns a string. 1 - "2" is -1, but 1 + "2" is "12", as the addition operator is used for string concatenation.
Parse the strings into integers:
var min = parseInt(prompt("Enter Min value:"), 10);

prompt returns a string so you need to convert min and max to a number:
var min = Number(prompt("Enter Min value:"));
var max = Number(prompt("Enter max value:"));

Simple Javascript Math Function- addition/not working?

This is my function:
var ans=(X*X)/(Y+Z);
When I enter 10, 20, and 10 - respectively- the addition bit comes out as 2010 and not 30.
How can I fix this?

Make sure to convert your strings to numbers first:
var X = "10";
var Y = "20";
var Z = "10";
X = +X; // unary plus operator converts to a number
Y = Number(Y); // or use the Number function
Z = parseInt(Z, 10); // or parseInt
var ans=(X*X)/(Y+Z);

Unexpected output in javascript

I am beginner to javascript and i am getting unexpected output
here is the code
<script type="text/javascript">
function add(a,b)
{
x = a+b;
return x;
}
var num1 = prompt("what is your no.");
var num2 = prompt("what is another no.")
alert(add(num1,num2));
</script>
it should give output as a sum of two number entered by us on prompting but it is simply concatenating the two number and popping the output

This is because the prompt function returns a String and not a Number. So what you're actually doing is to request 2 strings and then concatenate them. If you want to add the two numbers together you'll have to convert the strings to numbers:
var num1 = parseFloat(prompt("what is your no."));
var num2 = parseFloat(prompt("what is another no."));
or simpler:
var num1 = +prompt("what is your no.");
var num2 = +prompt("what is another no.");

prompt returns a string, not a number. + is used as both an addition and concatenation operator. Use parseInt to turn strings into numbers using a specified radix (number base), or parseFloat if they're meant to have a fractional part (parseFloat works only in decimal). E.g.:
var num1 = parseInt(prompt("what is your no."), 10);
// radix -----^
or
var num1 = parseFloat(prompt("what is your no."));

When you prompt the user, the return value is a string, normal text.
You should convert the strings in numbers:
alert(add(parseInt(num1), parseInt(num2));

The return value of prompt is a string. So your add function performs the + operator on 2 strings, thus concatenating them. Convert your inputs to int first to have the correct result.
function add(a,b)
{
x = parseInt( a ) + parseInt( b );
return x;
}

In addition to the already provided answers: If you're using parseInt() / parseFloat(), make sure to check if the input in fact was a valid integer or float:
function promptForFloat(caption) {
while (true) {
var f = parseFloat(prompt(caption));
if (isNaN(f)) {
alert('Please insert a valid number!');
} else {
return f;
}
}
}
var num1 = promptForFloat('what is your no.');
// ...

convert a JavaScript string variable to decimal/money

How can we convert a JavaScript string variable to decimal?
Is there a function such as:
parseInt(document.getElementById(amtid4).innerHTML)

Yes -- parseFloat.
parseFloat(document.getElementById(amtid4).innerHTML);
For formatting numbers, use toFixed:
var num = parseFloat(document.getElementById(amtid4).innerHTML).toFixed(2);
num is now a string with the number formatted with two decimal places.

You can also use the Number constructor/function (no need for a radix and usable for both integers and floats):
Number('09'); /=> 9
Number('09.0987'); /=> 9.0987
Alternatively like Andy E said in the comments you can use + for conversion
+'09'; /=> 9
+'09.0987'; /=> 9.0987

var formatter = new Intl.NumberFormat("ru", {
style: "currency",
currency: "GBP"
});
alert( formatter.format(1234.5) ); // 1 234,5 £
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/NumberFormat

This works:
var num = parseFloat(document.getElementById(amtid4).innerHTML, 10).toFixed(2);

An easy short hand way would be to use +x
It keeps the sign intact as well as the decimal numbers.
The other alternative is to use parseFloat(x).
Difference between parseFloat(x) and +x is for a blank string +x returns 0 where as parseFloat(x) returns NaN.

It is fairly risky to rely on javascript functions to compare and play with numbers. In javascript (0.1+0.2 == 0.3) will return false due to rounding errors. Use the math.js library.

I made a little helper function to do this and catch all malformed data
const convertToPounds = (str = "", asNumber = false) => {
let num = Number.parseFloat(str);
if (isNaN(num) || num < 0) num = 0;
if (asNumber) return Math.round(num * 1e2) / 1e2
return num.toFixed(2);
};
Demo is here

Prefix + could be used to convert a string form of a number to a number (say "009" to 9)
const myNum = +"009"; // 9
But be careful if you want to SUM 2 or more number strings into one number.
const myNum = "001" + "009"; // "001009" (NOT 10)
const myNum = +"001" + +"009"; // 10
Alternatively you can do
const myNum = Number("001") + Number("009"); // 10