By Daniel Ehrenberg. Slides at https://docs.google.com/presentation/d/1apPbAiv_-mJF35P31IjaII8UA6TwSynCA_zhfDEmgOE/edit#slide=id.p Quick, what do you get when you increment Math.pow(2, 53)? If you said Math.pow(2, 53), you may be a JavaScript programmer. From the beginning, JavaScript has supported 64-bit binary floats as its sole numeric type. In this talk, I’ll explain how, through TC39, JavaScript developers are working together with JS engine implementers and spec wonks like me to create BigInt: a native, unlimited-size integer type. Through collaboration, any layer of software can be changed, even the language itself. (c) JSConf EU 2018 June 2nd & 3rd 2018 — Berlin, Germany https://2018.jsconf.eu/

1. BigInts in JavaScript:
A case study in TC39
Daniel Ehrenberg
Igalia, in partnership with Bloomberg
TC39 delegate
JSConf EU 2018

2. const x = Math.pow(2, 53);

3. const x = 2 ** 53;

4. const x = 2 ** 53;
⇒ x === 9007199254740992

5. const x = 2 ** 53;
⇒ x === 9007199254740992
const y = x + 1;

6. const x = 2 ** 53;
⇒ x === 9007199254740992
const y = x + 1;
⇒ y === 9007199254740992

7. const x = 2 ** 53;
⇒ x === 9007199254740992
const y = x + 1;
⇒ y === 9007199254740992
WAT

8. This talk
● Why do Numbers do this?
● The fix: BigInt!
● How TC39 is developing BigInt
● What can you do to shape JS?

9. Number ===
64-bit float

10. Scientific notation
30000 = 3 × 105
sign exponent fraction
+1 5 3

11. IEEE 754 64-bit floats
From Wikimedia Commons, by Codekaizen, GFDL

12. 9007199254740992 ⇒
0100001101000000000000000000000000000000000000000000000000000000

13. 9007199254740992 ⇒
0100001101000000000000000000000000000000000000000000000000000000
9007199254740994 ⇒
0100001101000000000000000000000000000000000000000000000000000001

14. 9007199254740992 ⇒
0100001101000000000000000000000000000000000000000000000000000000
9007199254740994 ⇒
0100001101000000000000000000000000000000000000000000000000000001
Nothing in between!

15. Does this really
come up?

17. {"id":675127116845989888,
"id_str":"675127116845989888"}
===>
{"id":675127116845989900,
"id_str":"675127116845989888"}

20. > fs.lstatSync('one.gif').ino
// Underlying: 9851624185071827
9851624185071828
> fs.lstatSync('two.gif').ino
// Underlying: 9851624185071829
9851624185071828

22. const x = 2 ** 53;
⇒ x === 9007199254740992
const y = x + 1;
⇒ y === 9007199254740992

23. const x = 2n ** 53n;
⇒ x === 9007199254740992n
const y = x + 1n;
⇒ y === 9007199254740993n

24. const x = 2n ** 53n;
⇒ x === 9007199254740992n
const y = x + 1n;
⇒ y === 9007199254740993n

25. const x = 2n ** 53n;
⇒ x === 9007199254740992n
const y = x + 1n;
⇒ y === 9007199254740993n
nstands for BigInt

27. We are adding it
BigInt is at Stage 3
BOOM!

28. 67

29. // ES6 Destructuring Assignment
let x = 1, y = 2;
[y, x] = [x, y];
⇒ x === 2
y === 1

30. Who is TC39?
● A committee of Ecma, with…
● JS developers
● Implementers
● Frameworks
● Academics
● etc

33. Meetings
● Every two months
● For three days
● Summarize/review GitHub activity
● Move proposals through stages

34. TC39 stages
● Stage 1: An idea
● Stage 2: Agreed-on first draft
● Stage 3: Details worked out
● Stage 4: Battle-tested and standard

35. How did BigInt get
there?

36. Ancient history
● long/ulong types proposed (1999)
○ ES3 mostly library additions
○ ES4 abandoned
○ ES5, ES5.1 intentionally minimal
● ES6 proposed “value types”, including int64
○ ES6 deferred value types

37. Modern history
● Int64 to Stage 1, Nov 2016
● BigInt to Stage 1, Jan 2017
● Stage 3 in July 2017
● Shipped in Chrome 67, stable May 2018

38. Stage 1

41. BigInt vs Int64

42. 2n ** 53n + 1.5;
⇒ ????
TypeError!

43. Best practice for numeric types
● BigInt for integers which range > 253
● Number for always-small integers

44. Stage 2

48. Stage 3

49. ✔

52. Comparison change
● BigInt <-> String
comparison casts the
String to a BigInt
● Previously, sometimes
rounded as Number!
// Either way
0x10000000000000001n ==
"0x1000000000000001"
⇒ true
// Before change
0x10000000000000001n <
"0x10000000000000001"
⇒ true
// After change
0x10000000000000001n <
"0x10000000000000001"
⇒ false

53. BigInt to Stage 4?
● Full test262 tests ✔
● Specification PR ...
● 2+ implementations ...
● ES2019?

54. Getting involved

55. Feedback on GitHub issues
● Does this work for you?
● Missing pieces?
● Handle a case differently?

56. BigInt GitHub feedback
● Use cases from domain experts
● Proposal scope
● Int64 vs BigInt
● Semantic details

57. Test262 tests
● Tests shared between JS engines
● “If it’s not tested, it’s not compatible”
● Finds edge cases

59. BigInt test262 impact
● Made implementations correct the first time
● Fixed bugs, design
● 6+ test engineers collaborated, Bocoup and Igalia

60. Documentation/Education
● Explain the feature to contributors, learners
● Understand mental models for JS

61. Education insight from Ashley Williams
Folks new to programming have an intuition
about types
(Helped us decide on no-mixed-types semantics)

62. Implementation
● Open-source projects accepting contributions:
○ Babel
○ TypeScript
○ V8
○ Acorn
● Features developed behind a flag/in a plugin
○ JSC
○ ChakraCore
○ SpiderMonkey
○ And many more

63. BigInt implementation impact
● Get BigInt to JS developers
● Each implementation fixed bugs/design

64. Sponsorship
● Bloomberg -> Igalia’s spec, SpiderMonkey,
test262 work
● Google -> Bocoup’s test262 work

65. Attending TC39 as a delegate
● Join Ecma to come to TC39 meetings
● Contact me for more info, littledan@igalia.com

66. Take-aways

67. ● Get involved in TC39:
https://tc39.github.io/
● BigInt for ints ranging large
● Number for always-small ints
● Daniel Ehrenberg
● Twitter/GitHub @littledan
● littledan@igalia.com