Being at ConsenSys, I get to work with some of the best developers in the space. I recently published as a “Getting Started” guide for new developers who have stumbled into this magical ecosystem and don’t want to leave. It was not long until Joseph Chow raised issue with my neglecting to mention s and not longer after that until Jim Berry dropped a wonderful description of the danger at hand. So props to both of them and also thanks to Aakil Fernandes for explaining how works to me. this article Ethereum BigNumber BigNumber It turns out . I updated my previous article and felt compelled to write this one so that I could A) unburden my soul and B) show how may currently be mathing wrong between and . I’ve been doing things the wrong way you Javascript Ethereum This is important because . if you math wrong in Ethereum, you can lose money Numbers in Ethereum Much to the pain and suffering of many developers, Ethereum does not support floating point numbers at all. We are forced like barbarians to settle for s and s (I suggest you use the latter unless you have a very good reason to use the former) instead. There are probably valid technical reasons (I don’t know what they are) for this, but it’s fine because we can represent everything as integers and just keep track of decimals (recall that 1 ether = 10**18 wei). int uint Numbers in Javascript Javascript is weird. Because it’s a dynamically typed language, you can run into all sorts of interesting interpretation scenarios. The relevant problem here is that default division behavior in JS is (more specifically, double floating point division). This is incompatible with Ethereum, which (obviously) uses . floating point division integer division How to lose money Have you spotted the problem yet? Oh yes, Javascript’s [double] floating point numbers only go up to 16 decimals, but 1 ether = 10**18 wei. And . all ether values in transactions must be denominated in wei To see the problem in action, consider this: var my_money = 10*Math.pow(10, 18);my_money/3 > 3333333333333333500 Uh… whoops. You just gave me 167 wei. You may be saying “I don’t care about 167 wei.” And that’s a perfectly reasonable reaction. I don’t care about 167 wei either. But what happens when every one of the 1B daily state pushes that your swarm of IoT devices makes to is off by 167 wei? Your problems start adding up fast. Raiden BigNumber — A [really annoying] solution Contrary to popular belief, the built in function does absolutely for you. So we need to parse our number using some abstracted layer (I recommend , mostly because that’s what returns you anyway) and then operate on that. toPrecision() nothing bignumber.js web3.js So now our math looks like this: var my_money = 10*Math.pow(10, 18);var my_safe_money = new BigNumber(my_money); // Still wrongmy_safe_money/3> 3333333333333333500 // This is how you do itvar less_money = my_safe_money.div(3); So what do you do with this weird thing? BigNumber {[String: '3333333333333333333.333333333333333333']s: 1,e: 18,c: [ 33333, 33333333333333, 33333333333333, 33330000000000 ]} Well, you have to pass it around and keep operating on it with or or whatever. Look for the full list of functions. Unfortunately, if you ever pull out the value: div() times() here var free_at_last = less_money.toNumber(); You will be sad about the result: free_at_last> 3333333333333333500 So yeah… you have to keep doing operations. If you want a glimpse of what the number is, you can do this: BigNumber less_money.toString()> '3333333333333333333.333333333333333333' Also remember that if you’re forming transactions yourself, you have to provide Ethereum with , so you would need to do the following before putting it in a transaction: hex numbers less_money.floor().toString(16)> '2e426101834d5555' Of course if you’re abstracting that logic into web3, that will convert your numbers for you. Always Be Careful It’s hard out there for an Ethereum application developer (although not nearly as hard as it is for a Bitcoin application developer — I think those still exist?). We need to watch out for each other and be careful with what we’re doing. Always remember that there is (probably) money being moved by your application and take the necessary precautions. And, of course, if you find out about a pro tip or catch some dangerous behavior, report it! — If you like my articles, or check out my company, , because we do awesome stuff. follow me on twitter ConsenSys And, as always, there is an open invitation to join the and help us conquer the world. Ethereum community

A note on numbers in Ethereum and Javascript

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