Free email newsletter: “ES.next News

2014-02-08

What are integers in JavaScript?

According to the ECMAScript specification, all numbers in JavaScript are floating-point. Yet, the notion of integer comes up occasionally. This blog post explains what it means.

What are integers?

JavaScript has only floating-point numbers. Integers appear internally in two ways. First, most JavaScript engines store a small enough number without a decimal fraction as an integer (with, for example, 31 bits) and maintain that representation as long as possible. They have to switch back to a floating point representation if a number’s magnitude grows too large or if a decimal fraction appears.

Second, the ECMAScript specification has integer operators: namely, all of the bitwise operators. Those operators convert their operands to 32-bit integers and return 32-bit integers. For the specification, integer only means that the numbers don’t have a decimal fraction, and 32-bit means that they are within a certain range. For engines, 32-bit integer means that an actual integer (non-floating-point) representation can usually be introduced or maintained.

Ranges of integers

Internally, the following ranges of integers are important in JavaScript:
  • Safe integers [1], the largest practically usable range of integers that JavaScript supports:
    • 53 bits plus a sign, range (−253, 253)
  • Array indices [2]:
    • 32 bits, unsigned
    • Maximum length: 232−1
    • Range of indices: [0, 232−1) (excluding the maximum length!)
  • Bitwise operands [3]:
    • Unsigned right shift operator (>>>): 32 bits, unsigned, range [0, 232)
    • All other bitwise operators: 32 bits, including a sign, range [−231, 231)
  • “Char codes”, UTF-16 code units as numbers:
    • Accepted by String.fromCharCode()
    • Returned by String.prototype.charCodeAt()
    • 16 bit, unsigned

More blog posts on integers

All blog posts on integers have the label jsint. Converting to integer is covered by two blog posts:

References

  1. Safe integers in JavaScript
  2. Arrays in JavaScript
  3. Label bitwise_ops: blog posts on bitwise operators

No comments: