How to find the maximum number in javascript?

You can use Math.max

var FirstNumber = Number(prompt("Enter the First number"));

var SecondNumber = Number(prompt("Enter the Second number"));

var ThirdNumber= Number(prompt("Enter the third number"));

var ForthNumber = Number(prompt("Enter the forth number"));

var max = Math.max(FirstNumber, SecondNumber, ThirdNumber, ForthNumber);

document.writeln("<p>" + max + " is the largest number </p>");

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Note that this does not handle bad user input (like entering letters, scientific notation, etc). You should validate if you wanna avoid NaN (not a number) issues.

A solid way to format the user input is to use a formatter NumberFormat