참고: http://www.w3schools.com/js/js_obj_number.asp

All JavaScript Numbers are 64-bit

JavaScript is not a typed language. Unlike many other programming languages, it does not define different types of numbers, like integers, short, long, floating-point etc.

All numbers in JavaScript are stored as 64-bit (8-bytes) base 10, floating point numbers.

Precision

Integers (numbers without a period or exponent notation) are considered accurate up to 15 digits.

The maximum number of decimals is 17, but floating point arithmetic is not always 100% accurate:

var x;
document.write("
Only 17 digits: ");
x=12345678901234567890;
document.write(x + "
");

document.write("
0.2 + 0.1 = ");
x=0.2+0.1;
document.write(x + "
");

document.write("
It helps multiplying and dividing by 10: ");
x=(0.2*10+0.1*10)/10;
document.write(x +"
");

이에 대한 결과: 

Only 17 digits: 12345678901234567000

0.2 + 0.1 = 0.30000000000000004

It helps multiplying and dividing by 10: 0.3