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Murad Prokhorov
Murad Prokhorov

JavaScript Math.random() Explained: Examples and Tips.



Note: Math.random() does not provide cryptographically secure random numbers. Do not use them for anything related to security. Use the Web Crypto API instead, and more precisely the window.crypto.getRandomValues() method.




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Note that as numbers in JavaScript are IEEE 754 floating point numbers with round-to-nearest-even behavior, the ranges claimed for the functions below (excluding the one for Math.random() itself) aren't exact. If extremely large bounds are chosen (253 or higher), it's possible in extremely rare cases to reach the usually-excluded upper bound.


This example returns a random integer between the specified values. The value is no lower than min (or the next integer greater than min if min isn't an integer), and is less than (but not equal to) max.


Note: It might be tempting to use Math.round() to accomplish that, but doing so would cause your random numbers to follow a non-uniform distribution, which may not be acceptable for your needs.


Math.random() does not provide cryptographically secure randomnumbers. Do not use them for anything related to security. Use the WebCrypto API instead, and more precisely thewindow.crypto.getRandomValues() method.


Math.random() is fast and suitable for many purposes, but it's not appropriate if you need cryptographically-secure values (it's not secure), or if you need integers from a completely uniform unbiased distribution (the multiplication approach used in others answers produces certain values slightly more often than others).


To clarify the biased distribution concern, consider the case where we want to generate a value between 1 and 5, but we have a random number generator that produces values between 1 and 16 (a 4-bit value). We want to have the same number of generated values mapping to each output value, but 16 does not evenly divide by 5: it leaves a remainder of 1. So we need to reject 1 of the possible generated values, and only continue when we get one of the 15 lesser values that can be uniformly mapped into our target range. Our behaviour could look like this pseudocode:


Math.random() returns floating point number between 0 and 1 (like 0.344717274374 or 0.99341293123 for example), which we will use as a percentage, so Math.floor(Math.random() * 6) + 1 returns some percentage of 6 (max: 5, min: 0) and adds 1. The author got lucky that lower bound was 1., because percentage floor will "maximumly" return 5 which is less than 6 by 1, and that 1 will be added by lower bound 1.


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(following author's logic)Math.floor(Math.random() * 6) + 2, it is obviously seen that if we get 5 here -> Math.random() * 6 and then add 2, the outcome will be 7 which goes beyond the desired boundary of 6.


So, the correct logic is to take the difference between lower bound and upper bound add 1, and only then floor it (to substract 1, because Math.random() returns 0 - 0.99, so no way to get full upper bound, thats why we adding 1 to upper bound to get maximumly 99% of (upper bound + 1) and then we floor it to get rid of excess). Once we got the floored percentage of (difference + 1), we can add lower boundary to get the desired randomed number between 2 numbers.


Inspite of many answers and almost same result. I would like to add my answer and explain its working. Because it is important to understand its working rather than copy pasting one line code. Generating random numbers is nothing but simple maths.


You just need to add this to the head of your html document, and you can do pretty much whatever you want with randomness easily. Random values from arrays, random jquery elements, random properties from objects, and even preventing repetitions if needed.


The Javascript Math.random() function is used to return a floating-point pseudo-random number between range [0,1), 0 (inclusive), and 1 (exclusive). This random number can then be scaled according to the desired range.


Return Value: The math.random() function returns a floating-point, pseudo-random number between range [0,1) , 0 (inclusive) and 1 (exclusive). More codes for the above method are as follows


example 2: Math. random() can be used to get a random number between two values. The returned value is no lower than min and may possibly be equal to min, and it is also less than and not equal to max. For getting a random number between two values the math.random() function can be executed in the following way:


Example 4: Math.random() can be used to get an integer between a minimum and a maximum value, including not only the minimum but also the maximum. For getting a random integer between two values the Math.random() function can be executed in the following way:


Javascript is a full-featured programming language, able to make complex mathematical calculations quickly and accurately. The Javascript standard makes it easier to do these calculations with the Math object.


Computers are not capable of creating something truly random. True randomness is only possible through a source of external data that a computer cannot generate, such as the movement of many lava lamps at once (which has been used as an unbreakable random encryption in the real world), meteographic noise, or nuclear decay.


Javascript random numbers are not safe for use in cryptography because deciphering the seed could lead to decryption of the hidden number sequence. Some functions exist to create cryptographically secure pseudo-random numbers in Javascript, but they are not supported by older browsers.


Javascript creates pseudo-random numbers with the function Math.random(). This function takes no parameters and creates a random decimal number between 0 and 1. The returned value may be 0, but it will never be 1.


You can use Math.random() to create whole numbers (integers) or numbers with decimals (floating point numbers). Since Math.random() creates floating point numbers by default, you can create a random floating point number simply by multiplying your maximum acceptable value by the result from Math.random(). Therefore, to create a pseudo-random number between 0 and 2.5:


Creating a pseudo-random integer is a little more difficult; you must use the function Math.floor() to round your computed value down to the nearest integer. So, to create a random number between 0 and 10:


Fortunately, there are simple Javascript functions that programmers can create to make pseudo-random numbers more manageable. The rest of this section will show you how to create those functions, then put them all together into a single pseudo-random number generator.


Floating point numbers remain a little tricky here because Math.random() by default generates the maximum number of decimal places the Javascript implementation allows. In most circumstances, you probably want to cap your decimal places at 3 or 4 instead of reading the 10 or more that Math.random() usually creates.


Putting all of this together, a Javascript pseudo-random number generator that can create integers or floating point numbers with any number of decimal places could look like this. (Note that this implementation also includes error checking for the parameters.)


Cryptographically strong pseudo-random values are available on all web browsers that support Crypto.getRandomValues(). Implementations vary across user agents, but all are required to use a seed with enough entropy. To fill an array with results try:


As mentioned above, true random numbers must spring for a source outside computers without hardware dedicated to this purpose. For needs that require truly random cryptographically appropriate random numbers use one of the following external sources via their public APIs:


This is incredibly useful for gaming, animations, randomized data, generative art, random text generation, and more! It can be used for web development, mobile applications, computer programs, and video games.


The first line of code randomly shuffles the array and the second line returns a random umber between 0 and 10. In the example of a random color background, the range of colors and specifics such as hues, saturations, and shades can be set.


In this morphing fractal curve, Math.random is used twice to set the colors for the gradient and once more for the max radius of the curves. This is a great way to construct an entirely new appearance with every iteration!


This password generator uses Math.random to get a password array filled with uppercase and lowercase letters then adds random digits to the generated password. This is another great practical example!


Not exactly. Math.random() returns a pseudo-random number. This algorithm is called a pseudo-random number generator (or PRNG). This means its randomization can be reproduced under certain circumstances.


There are many methods to achieve unique values without repetition. The Fisher-Yates is one great way to prevent getting the same number twice by shuffling the sequence. Math.random will select a value from the shuffled array of a finite sequence demonstrated by the code snippet below.


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