Large numbers erroneously rounded in JavaScript

javascript floating-point floating-accuracy ieee-754
Jaanus picture Jaanus · Sep 4, 2009 · Viewed 32.3k times · Source

See this code:

When I see my console in Firefox 3.5, the value of jsonParsed is the number rounded:

Object id=714341252076979100 type=FUZZY

Tried different values, the same outcome (number rounded).

I also don't get its rounding rules. 714341252076979136 is rounded to 714341252076979200, whereas 714341252076979135 is rounded to 714341252076979100.

Why is this happening?

Answer

Stephen Canon picture Stephen Canon · Sep 4, 2009

What you're seeing here is actually the effect of two roundings. Numbers in ECMAScript are internally represented double-precision floating-point. When id is set to 714341252076979033 (0x9e9d9958274c359 in hex), it actually is assigned the nearest representable double-precision value, which is 714341252076979072 (0x9e9d9958274c380). When you print out the value, it is being rounded to 15 significant decimal digits, which gives 14341252076979100.

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