Daniel Ehrenberg, Igalia. Stage 3

Thanks for help and feedback on this effort from Brendan Eich, Waldemar Horwat, Jaro Sevcik, Benedikt Meurer, Michael Saboff, Adam Klein and others.

Find the nth prime:

// Takes a BigInt as an argument and returns a BigInt
function nthPrime(nth) {
  function isPrime(p) {
    for (let i = 2n; i < p; i++) {
      if (p % i === 0n) return false;
    }
    return true;
  }
  for (let i = 2n; ; i++) {
    if (isPrime(i)) {
      if (--nth === 0n) return i;
    }
  }
}

Read and add two unsigned 64-bit integers in asm.js on the heap:

function Add64Module(stdlib, foreign, buffer) {
  "use asm";
  var cast = stdlib.BigInt.asUintN;
  var values = new stdlib.BigUint64Array(buffer);
  function add64(aIndex, bIndex) {
    aIndex = aIndex|0;
    bIndex = bIndex|0;
    var aValue = values[aIndex>>3];
    var bValue = values[bIndex>>3];
    return cast(64, aValue + bValue);
  }
  return { add64: add64 };
}

In many cases in JavaScript coding, integers larger than 2⁵³ come up, where casting to a double-precision float would lose real, relevant data:

• Cases which would fit in a signed or unsigned 64-bit integer
  • Reading certain machine registers, wire protocols
  • Protobufs or JSON documents that have GUIDs in them
  • stat may give some data as 64-bit integers
  • Accurate timestamps
• Bigger than 64-bit int cases
  • Generally meeting a reasonable user expectation of a high-level language that integer arithmetic will be "correct" and not suddenly overflow
  • Basis for implementing BigDecimal; perhaps some applications would be happy with fixpoint calculations based on a BigInt
  • Any mathematical calculation with larger integers, e.g., solving Project Euler problems
  • Exact geometric calculations

This proposal provides a new, second primitive numeric type, BigInt, to meet those use cases.

BigInts are a new numerical type supporting literals (1234n) and arithmetic with operator overloading (1n + 2n makes 3n).

BigInts are, logically, arbitrary mathematic integers, with operator definitions which fall out naturally:

• Binary +, -, * and ** find their mathematical answer when applied to two BigInts
• / and % round towards 0
• Bitwise operations |, &, <<, >>, ^ operate logically
  • Negative numbers to be interpreted as infinite-length two's complement (see discussion).
• When applied to two BigInts, comparison operators ==, ===, <, >, >=, and <= perform a mathematical comparson
• Missing operators
  • >>> is not supported, as all BigInts are signed; to get an unsigned shift, pass in a positive BigInt to >> (discussion)
  • Unary + is unsupported on BigInts due to asm.js requirements; details explained below
• In a conditional, if (0n) executes the else branch.

Literals for BigInts are similar to Number literals, but followed by n. They can be written with binary, octal or hexadecimal notation, e.g., 0x100n. Legacy octal syntax (0640) is not allowed, only new-style (0o064n).

The choice of BigInt comes from an attempt to preserve user intuition about the lack of compatibility of BigInt with things that are currently based on Numbers. Because implicit coercisions lead to TypeErrors, the guidance is that existing users of Numbers should stay with Numbers if it works for the application, and only large usages need to upgrade to BigInt. The name BigInt is hoped to emphasize this not-by-default usage. The suffix n is basically arbitrary.

BigInts are a primitive type, and BigInt forms a constructor which can be used analogously to Number--to convert various types to BigInt values, as well as to be an object wrapper for BigInt values when used with property access.

When called as a function, it is similar to the Number constructor: It converts strings, Numbers, etc into BigInts.

• BigInt.asUintN(width, BigInt): Wrap a BigInt between 0 and 2^width-1
• BigInt.asIntN(width, BigInt): Wrap a BigInt between -2^width-1 and 2^width-1-1
• BigInt.parseInt(string[, radix]): Analogous to Number.parseInt, to parse a BigInt from a String in any base.

BigInts give JavaScript the ability to accurately represent 64-bit signed and unsigned integers:

• BigUint64Array and BigInt64Array, whose elements read from property access are BigInts
• DataView.prototype.getBigInt64/getBigUint64, returning a BigInt

Similarly, BigInts may be used by the WebAssembly FFI for 64-bit arguments and return values to functions.

A key design decision is to disallow mixed operations between BigInts and Numbers. The driving factor: Any implicit coercions would lose information.

When adding two values of different numeric types, between large integers and floating point numbers, the mathematical value of the result may be outside of the domain of either. For example, (2n**53n + 1n) + 0.5 has a value which cannot be accurately represented in either range. Floating point arithmetic is not the exact mathematical value, but at least it is well-defined by IEEE 754. The entire aim of introducing BigInt is, on the other hand, to make a way to preserve integer precision of larger values.

Many (all?) other dynamically typed programming languages which have multiple numeric types implement a numeric tower. This forms an ordering between types--on the built-in numeric types, when an operator is used with operands from two types, the greater type is chosen as the domain, and the "less general" operand is cast to the "more general" type. Unfortunately, as the previous example shows, there is no "more general" type between arbitrary integers and double-precision floats. The typical resolution, then, is to take floats as the "more general" type.

Silently losing precision sometimes may be a problem, but in most dynamically typed programming languages which provide integers and floats, integers are written like 1 and floats are written like 1.0. It's possible to scan code for operations which may introduce floating point precision by looking for a decimal point. JavaScript exacerbates the scope of losing precision by making the unfortunate decision that a simple literal like 1 is a float. So, if mixed-precision were allowed, an innocent calculation such as 2n ** 53n + 1 would produce the float 2**53--defeating the core functionality of this feature.

To avoid this problem, this proposal bans implicit coercions between Numbers and BigInts, including operations which are mixed type. 1n + 1 throws a TypeError. So does passing 1n as an argument into any JavaScript standard library function or Web API which expects a Number. Instead, to convert between types, an explicit call to Number() or BigInt() needs to be made to decide which domain to operate in. 0 === 0n returns false.

Comparisons form an exception to this rule: It's mathematically well-defined to allow comparison operators such as < and == compare between Numbers and BigInts. Unlike operators like +, there is no loss of precision, since the output is just a Boolean. Further, == is extended in comparisons with strings in analogous ways, such as 0n == "" is true. Although we may not be able to extend this support to user-defined types, it seems sufficiently important for meeting user expectations that this proposal adds them as a one-off.

When a messy situation comes up, this proposal errs on the side of throwing an exception rather than silently giving a bad answer. This is what's behind throwing a TypeError on adding a BigInt and a Number: If we don't have a good answer, better to not give one.

Some JavaScript users will surely have the intuition that everything will just work, and that exceptions will not be thrown, when doing this sort of interoperation, even at the cost of losing precision. Axel Rauschmeyer elaborated a proposal which elaborates this approach. Further discussion raised a few issues with this approach which are a direct result of upgrading the behavior of existing Numbers, both in terms of compatibility and reasonable expectations of invariants of JavaScript values. For now, this repository will stick with the distinct type approach.

The semantics of all operators should ideally be based on some mathematical first principles, and certainly be well-defined, rather than excessively exposing implementation artifacts. / and % round towards 0, this is to match well-established computer conventions; aside from that, all operators have clean, mathematical definitions which don't appeal to the implementation shape.

Although this proposal introduces operator overloading, it throws in any of the cases that asm.js depends on for setting up type checking. asm.js relies on a few identies:

• Unary + followed by an expression is always either a Number, or results in throwing. For this reason, unfortunately, + on a BigInt needs to throw, rather than being symmetrical with + on Number: Otherwise, previosly "type-declared" asm.js code would now be polymorphic.
• |0 always returns a Number in int32 range, or throws. This proposal maintains that, as it would throw on a BigInt for being a mixed operand type.
• Math.fround always returns a Number in float32 range, or throws. This proposal would throw if Math.fround is called with a BigInt, preserving the property.

Analogously, >>> 0 always returns a Number in uint32 range, throwing as >>> is not supported on BigInt at all. Note: asm.js itself does not require this property, as >>> may be an overloaded operator, and |0 is used for all int parameter declarations, but >>> 0 is a common idiom to achieve this property in JavaScrpit code.

This proposal makes special allowances to make BigInt usable in asm.js code to build support for 64-bit integers, by including the standard library functions BigInt.asUintN and BigInt.asIntN as well as BigUint64Array and BigInt64Array. The operator overloading in this proposal should not complicate the asm.js model: asm.js already treats operators as "overloaded" between floats, doubles, and signed and unsigned integers.

• Should pave the cowpath to value types, as previously discussed, in conjunction with the work done on SIMD.js.
• BigInts are a new primitive type, and have associated wrappers, as do the other primitives, and SIMD.js, and as value types would get.
• Operator overloading on value types may follow a similar pattern of requiring uniform argument types; this avoids the very difficult proposition of double dispatch. By not supporting mixed operands, BigInt gets no superpowers which would be very difficult to generalize. Mixed comparisons are a one-off exception to this principle, however.
• L has been proposed as a literal suffix for positive Int64 values. This proposal uses n to leave that space free for later (bikeshedding welcome!).

This proposal comes with built-in operator overloading in order to not make BigInts too ugly to be usable. One particular hazard, if BigInts were to be operated on with static methods, is that users may convert the BigInt into a Number in order to use the + operator on it--this would work most of the time, just not with big enough values, so it might pass tests. By including operator overloading, it would be even shorter code to add the BigInts properly than to convert them to Numbers, which minimizes the chance of this bug.

This proposal adds a new primitive type with wrappers, similar to Symbol. As part of integrating BigInts into the JavaScript specification, a high amount of rigor will be required to differentiate three types floating around in the specification: Mathematical values, BigInts and Numbers.

We need to choose a web-compatible name to add to the global object. There is some worry that the name Integer could have a web compatibility risk, though we don't actually have data about this. Changing the semantics of existing operations performed on existing numbers might also have compatibility risk, and this proposal avoids making these kinds of changes.

Design work here is being done in conjunction with planned prototyping in V8; this will be used to develop feedback to ensure that the proposal is efficiently implementable.

Brendan Eich previously proposed two types--signed and unsigned Int64/Uint64, for JavaScript. These meet many of the concrete use cases for BigInts. One claim is that they may provide more predictable performance; however, my understanding is that, if the appropriate casting operator (e.g., BigInt.asUintN) is used everywhere, an implementation like V8 is expected provide the same performance for BigInt as it would for an Int64 type. The risks of this approach are that the performance won't pan out without harder-to-remove cliffs, or that the ergonomics will be too bad for performance-sensitive code--we'll watch for these risks as the prototype advances.

We could allow mixed operands like 1 + 1n returning 2 (a Number). It would lose precision, be harder to implement with as high performance, and not generalize well to user-defined types, but it would follow the well-worn path of many other programming languages, which have just let users deal with these issues.

One possibility would be to wait until a potential future Int64/Uint64 proposal is created for TypedArray classes and DataView methods. However, they are included here, following the pattern of existing TypedArrays which return Numbers--a more general type than the contents, but one which accurately represents it.

It would be reasonable to add integer-related mathematical library functions, especially those which could be more efficiently based on instructions found on CPUs likely to be used by implementations. This includes:

• Bitcast between BigInt and Number
• Find first set/unset bit
• Popcount
• Find most significant set/unset bit
• Convenience functions for doing arithmetic in a specific modulus (e.g., 64-bit signed or unsigned) rather than requiring use of the wrap functions and arithmetic separately.
• Constants for the maximum and minimum 64-bit signed and unsigned integer

In the course of development of ES2015, the proposal to add 64-bit integers was generalized significantly into a value types/binary data proposal. This big proposal became very complicated and unwieldy, so it was dropped from ES2015. The current proposal is much smaller, and could be built on incrementally. Value types and smaller integer types may be possible follow-ons, and this proposal is designed to generalize to those, but not block on them.

No current (known) implementations exist.

See the specification for more fine details.