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:

• Converting to integer via shift operators: “Integers and shift operators in JavaScript”
• Converting to integer via parseInt(): “parseInt() doesn’t always correctly convert to integer”

References

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