[clxl]

Excel only supports numbers that fit in an IEEE754 double, just like JavaScript.

There is some fancy logic to format numbers, however (including some mess for date codes). http://oss.sheetjs.com/ssf/ is a pretty decent implementation in JavaScript.

[brudgers]

All versions of Excel have native support for integers.

http://msdn.microsoft.com/en-us/library/office/bb687869%28v=...

[clxl]

Excel supports 32 bit integers, which fit comfortably in IEEE754 doubles (there are 64 bit integers which cannot be exactly represented)