BigDecimal: Avoid rounding errors on decimals in JavaScript (Node.TLV 2020)

@littledan
Decimal: Avoid Rounding
Errors on in JavaScript
Daniel Ehrenberg
Igalia
Node.TLV 2020

@littledan@littledan
‫אני‬ ‫מי‬
● Daniel Ehrenberg
● @littledan
● Delegate in TC39

‫שלי‬ ‫הבית‬
Les Roquetes del Garraf, Europe

‫שלי‬ ‫ה‬ ָ‫בר‬ֶ‫ח‬
Embedded WebKit and Chromium development
Mesa and GStreamer drivers
CSS, ARIA, WebAssembly, MathML, JavaScript
standards+implementation in web browsers+Node.js

Who is TC39?
● A committee of Ecma, with…
● JS developers
● JavaScript engines
● Transpilers
● Frameworks, libraries
● Academics
● Node.js collaborators
● Some big websites/app platforms
● (No Israeli companies ☹)

Meetings
● Every two months
● For three days
● Discuss language changes
● Seek consensus on proposal
Stage advancement

TC39 stages
● Stage 1: An idea under discussion
● Stage 2: We want to do this, and we have a ﬁrst draft
● Stage 3: Basically ﬁnal draft; ready to go
● Stage 4: 2+ implementations, tests ⇒ standard

Consensus-based decision-making
● TC39 doesn't vote on what the language will be
● TC39 is consensus-seeking
○ We work together to meet everyone's goals
○ "Does anyone object to stage advancement?"
○ Objections must have rationale, appropriate to stage
● Goals: Listen to everyone engaging in process;
Don't choose one stakeholder over another as the "decider";
Avoid standardizing things which aren't ready yet

const x = 2 ** 53;
⇒ x === 9007199254740992
const y = x + 1;
⇒ y === 9007199254740992
@littledan

const x = 2n ** 53n;
⇒ x === 9007199254740992n
const y = x + 1n;
⇒ y === 9007199254740993n
@littledan

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

67 🚢 68 🚢
Flag 🏗
79 🚢
10.4 🚢

Igalia's role in BigInt
● Wrote BigInt speciﬁcation and some tests
● Standardized BigInt in TC39
● Implemented BigInt in SpiderMonkey (shipping)
and JSC (in progress)

@littledan
Private ﬁelds and
methods:
Stage 3

Why?
● Private methods encapsulate
behavior
● You can access private ﬁelds inside
private methods
class Counter extends HTMLElement {
#x = 0;
connectedCallback() {
this.#render();
}
#render() {
this.textContent =
this.#x.toString();
}
}

#
is the new
_
for strong encapsulation

class PrivateCounter {
#x = 0;
}
let p = new PrivateCounter();
console.log(p.#x); // SyntaxError
class PublicCounter {
_x = 0;
}
let c = new PublicCounter();
console.log(c._x); // 0
Stage 3
@littledan

Why not private keyword?
● In languages with types:
obj.x can check whether x is private by looking at the type of obj
● JavaScript is dynamically typed
● ⇒ private vs public distinction needed at access point,
not just deﬁnition
● # as part of the name was the cleanest, simplest solution we found
● We thought about many alternatives over 20 years; ask me later about any
further possibilities

@littledan
Stage 2 features
We want to do this, and we have a ﬁrst draft

@littledan
Decorators: Stage 2
Yehuda Katz
Ron Buckton

// Salesforce abstraction
import { api } from '@salesforce';
class InputAddress extends HTMLElement {
@api address = '';
}
Using decorators to track mutations and rehydrate
a web component when needed.
Syntax abstraction for attrs/props in Web Components
// Polymer abstraction
class XCustom extends PolymerElement {
@property({ type: String, reflect: true
})
address = '';
}
Example of using decorators to improve ergonomics.

@littledan
Stage 1 features
An idea under discussion

Problem and solution (?)
// Number (binary 64-bit floating point)
js> 0.1 + 0.2
=> 0.30000000000000004
// BigDecimal or Decimal128 (???)
js> 0.1m + 0.2m
=> 0.3m

What's wrong with Number?
● Number is an IEEE 754 64-bit ﬂoat

Number: an IEEE 754 64-bit ﬂoat
From Wikimedia Commons, by Codekaizen, GFDL

What's wrong with Number?
● Number is an IEEE 754 64-bit ﬂoat
● Base 2 fractions can't represent .1 or .2 exactly!
> .1.toPrecision(70)
<< "0.1000000000000000055511151231257827021181583404541015625000000000000000"
> .2.toPrecision(70)
<< "0.2000000000000000111022302462515654042363166809082031250000000000000000"
● ⅓ in base 10: closest we can get is .3333333...,
but eventually we have to cut it oﬀ

Use cases and requirements for decimal
Primarily: Dealing with human-written decimal quantities, like money
● Represent typical quantities precisely
● Arithmetic operations: + - * /
● Rounding w/ conﬁgurable precision, mode
● Serialize/deserialize with strings
● Display to users
● Easy to use correctly

Why add this to JavaScript now?
We got by without JS decimal
● Server in another language
● JS fronts a thin client
● Don't trust the client anyway (still true!)
What changed?
● Richer clients doing local calculations
(deﬁnitively calculated on server, but should avoid ephemeral inaccuracies)
● Server-side in JS with serverless and Node.js!

@littledan
How JS decimal
would work

Basic usage
● Literal syntax:
123.456m
● Operator overloading:
.1m + .2m === .3m

Serialization: JSON.stringify() and toString()
● toJSON like BigInt: Throws by default :(
○ Sorry, changing JSON would break compatibility
○ JSON.stringify takes second parameter to customize
JSON.stringify(123.456m) // ⇒ TypeError
● toString like Number and BigInt:
○ Excludes the m suﬃx
(123.456m).toString() // ⇒ "123.456"

Presentation to the user: Intl.NumberFormat

Presentation to the user: Intl.NumberFormat
● Intl.NumberFormat.prototype.format, formatToParts:
overloaded for BigDecimal
(like BigInt)
● BigDecimal.prototype.toLocaleString works too
(123123.456m).toLocaleString("de-DE") // ⇒ "123.123,456"

Rounding: the .round(decimal, options) method
● roundingMode: e.g., "up", "down", "half-up", "half-even" (more?). Required
explicitly
● Precision options (names matching Intl.NumberFormat, only one permitted):
○ maximumFractionDigits
○ maximumSigniﬁcantDigits
BigDecimal.round(3.25m, { roundingMode: "half-up",
maximumFractionDigits: 1 })
====> 3.3m

@littledan
The data model. Options:
● Fraction
● BigDecimal
● Decimal128

Rationals, i.e., Fractions
Common Lisp, Scheme, OCaml, Ruby, Perl6 etc.
● 2.53m => 253/100
● ⅓ represented exactly
● No need to round for any arithmetic!
numerator
denominator
BigInt…………………...
BigInt…………………...

In this proposal, we're not pursuing rationals
Core operations for the primary use case are not a good ﬁt
● Rounding to a certain base-10 precision, with a rounding mode
○ "Round up to the nearest cent after adding the tax"
● Conversion to a localized, human-readable string
○ "And display that to the user"
● Fractions may be proposed separately
○ E.g., Python and Ruby have both fractions and decimal
○ Just diﬀerent use cases, no one subsumes the other

Fixed-size decimal, e.g., Decimal128
Python, C#, IEEE 754-2008, Swift, Decimal.js
● Pick a maximum number of digits
● Floating point, only in base 10
● Rounds if there isn't enough space
e.g. 32.5 ->
Value: sign * mantissa * 10exponent
sign (1 or -1)
exponent base 10
mantissa: decimal fraction
0
2
.325

Arbitrary-size decimal, i.e., BigDecimal
Ruby, Java, Big.js
● Number of digits grows with the number
● Represent (almost) any decimal exactly
● Avoid rounding*
Value: sign * mantissa * 10exponent
sign
exponent
mantissa BigInt…………………...

Troubles with precision and BigDecimal
● What should 1m/3m be?
● How long should the decimal go?
● Option: Cut it oﬀ at an arbitrary point (Ruby)
○ .33333333333333m
● Option: Force a rounding options parameter (Java)
○ BigDecimal.div(1m, 3m, { roundingMode: "half-up",
maximumFractionDigits: 2 })

Plan: Why not both?
● There are pros and cons of BigDecimal and Decimal128
● Our plan: Try both and gather feedback
○ Implement two polyﬁlls and write full documentation
○ Encourage people to try it out in their programs
○ Collect feedback through surveys, GitHub issues
● Results ⇒ Stage 2 proposal

@littledan
Participating in TC39

Helping with existing proposals
● Often highly appreciated!
● What you want may already be a proposal
○ You might not be the only person with this problem
How you can help: participating on GitHub, with a proposal at a stage...
● Stage 0/1: Help determine use cases, very high level design
● Stage 2: Help work out all the details, early toy implementations
● Stage 3: Implement, document and test
● Stage 4: It's already done

Participating internationally
● Many TC39 members are in a similar situation to you all:
○ English is a second language
■ You can participate with mostly written communication
○ Live outside the US
■ Many delegates live in Europe
○ Unable to attend most TC39 meetings in person
■ Instead, join by video call
● TC39's code of conduct prohibits discrimination on nationality
● Many TC39 members want to increase international participation
● TC39 participants represent ideas and organizations, not countries

Participating asynchronously on GitHub
● Most tasks don't take place in meetings:
Most important technical work happens on GitHub
● Work on GitHub:
○ Some of the discussion about design
○ Speciﬁcation text
○ Documentation
○ Tests
○ Implementations
● Non-members can contribute on GitHub! We encourage it.
○ Just sign non-member IPR form
● Meeting notes are published to GitHub
● Only members can take part in meetings and consensus process

Joining TC39
● Join TC39 by joining Ecma
● Joining Ecma requires:
○ Signing IPR forms
■ Typical Ecma RAND policy
■ TC39-speciﬁc royalty-free agreement
○ Paying membership fee
○ Technically, a vote (typically a formality)
● TC39 delegates represent member organizations
● Companies considering joining can provisionally attend TC39 as
"prospective members"

@littledan
TC39 changes over time

Older TC39 mode
● Speciﬁcation: Big MS Word doc
● Communication: Meetings and es-discuss list
● Large, occasional speciﬁcation; no stages

JS Decimal's future in this context
● Aim to be participatory, rigorous and experience-driven
● Prototyping multiple alternatives: BigDecimal and Decimal128
● Developing proposals to generalize decimal:
Operator overloading, extended numerical literals
● This is still early, and we could use your help

@littledan
!‫תודה‬
Follow up on Decimal:
github.com/tc39/proposal-decimal
Tell us your Decimal use cases:
bit.ly/2PFDT2o
Get involved in TC39:
tc39.es
Stay in touch with me:
@littledan on Twitter (DMs open)
littledan@igalia.com
Learn more about Igalia: igalia.com

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