Be careful with JS numbers!


It is common in Javascript to have unexpected behaviors, but this one is particulary vicious.

10000000000000000 === 10000000000000001

Javascript doesn’t have integer type but lets you think it has. parseInt and parseFloat built-in functions, the fact that “1″ is displayed as “1″ and not as “1.0″ (like many languages) contribute to the general misunderstood.

In Javascript, all numbers are floating numbers and are prone to floating point approximation.

When you write var i = 1;, and you console.log it, Javascript is nice, you obtain 1 and not 1.0000000000000001.

But you can experiment that, in Javascript, 1.0000000000000001 === 1 is true…

I hear you, telling me that this sounds OK, floating point approximation rules, right?

But the same thing occurs for big numbers:

10000000000000000 === 10000000000000001

Oh F**K !

[edit] where in python:

Termination of loops

The following is worse:

is logging 10000000000000000 forever!

Because 10000000000000001 can’t exist in Javascript with approximations, 10000000000000001 is 10000000000000000, so you can’t increment this value, and you are stuck in this crazy f**king loop.

Conclusion, Program termination proof sounds hard to reach in Javascript!

How many numbers in a 1000 range?

Between 10000000000000000 and 10000000000001000, there are actually 750 Javascript integers.

Real World Example

The issue can actually lead to real web application disaster. Imagine if your database use Long for id (well like almost every databases in the world, like twitter does), and if you use the id as number in Javascript and not as string, you can have strange behaviors like never being able to represent and access a resource from the Javascript or worse!

TL;DR. The lesson

This is not something new, floating point approximation, but the way Javascript fix values to round the approximations mislead us.

Now, simple thing, Avoid numbers when approximation is not permitted like for resource id (especially when you retrieve it from a server).

This probably impacts your JSON APIs because it’s the last thing you had think of!

Otherwise, if you need to manipulate big integers in Javascript use a library for that.


9007199254740993 (which is 2^53 1) is the smallest not representable integer in Javascript. In other words, you can trust Javascript numbers before this integer!

[EDIT 2]
Thanks to 0×0 on HackerNews who told me the twitter id issue example really happened in a previous twitter API:

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