How does V8 represent numbers?

The language has one Number type: the spec defines it as an IEEE 754 double-precision value. Smi and HeapNumber are V8's internal representations (observed on Node 24.19, V8 13.6, 64-bit arm64); you cannot see them from JavaScript semantics, only from performance and debug tooling.

Tagging. V8 values are tagged words. Per the pointer-compression post, the least significant bit(s) distinguish a heap-object pointer from a small integer (Smi). A Smi carries its value in the word itself, so it needs no heap allocation and no pointer to follow.

Range. The post says a Smi has a 32-bit payload on 64-bit builds and 31 bits when pointer compression is on, because compression makes the tagged word 32 bits wide. On this Node, process.config.variables.v8_enable_pointer_compression is 0, and %IsSmi was true for 2**31 - 1 and -(2**31) and false one step beyond either. So the range here is the 32-bit one. A browser such as Chrome ships with compression on, per the post, so I would expect 31 bits there; I did not test it.

HeapNumber. Any number that does not fit a Smi is a heap object holding a boxed double: fractions, -0, NaN, Infinity, and integers outside the Smi range. In %DebugPrint, 1.5 is a [HeapNumber] with instance size: 16 here.

Consequences.

• Whether a number is a Smi depends on its value, not on how you wrote it: 1.0 and 10 / 2 are Smis, 7 / 2 is not.
• Arrays hold doubles unboxed in PACKED_DOUBLE_ELEMENTS (elements kinds), so a double array does not need one HeapNumber per element.
• Optimized code that expects Smis deoptimizes when it meets a HeapNumber (not a Smi) or an overflowing result (overflow) (deoptimization).

// smi.mjs. Run: node --allow-natives-syntax smi.mjs (Node 24.19, V8 13.6, 64-bit,
// pointer compression off). %IsSmi is a debug-only native, not public API.
const MAX = 2 ** 31 - 1;
console.log(%IsSmi(MAX), %IsSmi(MAX + 1)); // → true false
console.log(%IsSmi(-(2 ** 31)), %IsSmi(-(2 ** 31) - 1)); // → true false
console.log(%IsSmi(1.0), %IsSmi(1.5)); // → true false
console.log(%IsSmi(-0), %IsSmi(NaN)); // → false false
console.log(%IsSmi(10 / 2), %IsSmi(7 / 2)); // → true false
%DebugPrint(1.5); // → [HeapNumber] ... - value: 1.5 ... - instance size: 16

• Quoting "Smis are 31-bit" or "32-bit" without the build. It is 31 with pointer compression and 32 without, per the V8 post. On this Node it is 32.
• Believing JavaScript has an integer type. 1.0 === 1; the engine decides the representation per value.
• Assuming -0 is an integer. %IsSmi(-0) was false; -0 needs a double.
• Treating the Smi range as a language limit. Numbers beyond it work, as HeapNumbers. Also, %IsSmi and %DebugPrint are debug-only natives, not stable.