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I know that I need to use Math.random() for making random numbers, but today I tried to make a random number between 1 and 9999...(9 repeated 19 times) and my output always ends in 3-5 zeroes. How can I generate more detailed random numbers?

What I've done:

const foo = Math.floor(Math.random() * parseInt("9".repeat(19)));

Also, I'm pretty sure I know how to do this, but if anyone can tell me, how do I pad zeroes to get to a certain digit count? (ex. pad(15,4) bees 0015 because the you need 2 more digits to make it 4 digits long)

I know that I need to use Math.random() for making random numbers, but today I tried to make a random number between 1 and 9999...(9 repeated 19 times) and my output always ends in 3-5 zeroes. How can I generate more detailed random numbers?

What I've done:

const foo = Math.floor(Math.random() * parseInt("9".repeat(19)));

Also, I'm pretty sure I know how to do this, but if anyone can tell me, how do I pad zeroes to get to a certain digit count? (ex. pad(15,4) bees 0015 because the you need 2 more digits to make it 4 digits long)

• javascript
• random
• digits
• zero-padding

Share Improve this question Follow asked Jan 28, 2019 at 22:48 DexieTheSheepDexieTheSheep 46011 gold badge77 silver badges1616 bronze badges 3

• 7 The maximum integer you can represent without loss of precision is 2^53 - 1, which has 16 digits. In other words, you cannot have an integer value derived from a 19 digit long string without loosing precision. Proof: 9007199254740991 + 1 and 9007199254740991 + 2 are both 9007199254740992. Wele to the world of floating point values. – Felix Kling Commented Jan 28, 2019 at 22:53
• 1 So, what you may want is Math.floor(Math.random() * Number.MAX_SAFE_INTEGER); instead. – Felix Kling Commented Jan 28, 2019 at 22:57
• padStart – Bergi Commented Jan 28, 2019 at 22:58

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3 Answers 3

Sorted by: Reset to default 3

The best idea is probably to just use a string of random integers (solves padding too):

let foo = '';
for(i=0; i<19; ++i) foo += Math.floor(Math.random() * 10);
alert(foo);

You are running into Number.MAX_SAFE_INTEGER. The largest exact integral value is 2^53-1, or 9007199254740991.

You need to use numbers encoded as strings. A loop like this:

var desiredMaxLength = 19
var randomNumber = '';
for (var i = 0; i < desiredMaxLength; i++) {
    randomNumber += Math.floor(Math.random() * 10);
}

Arthimetic for numbers represented as strings can be donw with the strint library found at https://github./rauschma/strint.

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