Precise Financial Calculation in JavaScript. What Are the Gotchas?

You should probably scale your decimal values by 100, and represent all the monetary values in whole cents. This is to avoid problems with floating-point logic and arithmetic. There is no decimal data type in JavaScript – the only numeric data type is floating-point. Therefore it is generally recommended to handle money as 2550 cents instead of 25.50 dollars.

Consider that in JavaScript:

var result = 1.0 + 2.0;     // (result === 3.0) returns true

But:

var result = 0.1 + 0.2;     // (result === 0.3) returns false

The expression 0.1 + 0.2 === 0.3 returns false, but fortunately integer arithmetic in floating-point is exact, so decimal representation errors can be avoided by scaling¹.

Note that while the set of real numbers is infinite, only a finite number of them (18,437,736,874,454,810,627 to be exact) can be represented exactly by the JavaScript floating-point format. Therefore the representation of the other numbers will be an approximation of the actual number².

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¹ Douglas Crockford: JavaScript: The Good Parts: Appendix A – Awful Parts (page 105).
² David Flanagan: JavaScript: The Definitive Guide, Fourth Edition: 3.1.3 Floating-Point Literals (page 31).