proposal-decimal—A polyfill for the decimal proposal (exact decimal numbers for JavaScript)

This library is a prototype for the decimal proposal. There should be no observable difference between what this library does and what the proposal is supposed to do. If you find a mismatch between what this code does and what the decimal proposal says it should do, please file an issue in this repo.

This package is a prototype and is not intended for production use. (The decimal proposal is currently at Stage 1 in the Ecma TC39 process.) Speed is important, but it is less important than making sure we follow the spec exactly. However, if you notice that this package is significantly slower than other decimal implementations, please file an issue.

Data model

There are a few different ways out there to model decimal numbers. The decimal proposal currently intends to use the IEEE 754 Decimal128 decimal floating-point standard. There is one twist, though: we do not expose the precision (or, officially, quantum) of Decimal128 values. A consequnce of this decision is that we do not support trailing zeroes. Thus, the conversion from "1.2" and "1.20" to Decimal128 values yields distinct values in the official IEEE 754 Decimal128 universe, bot for the purposes of the decimal proposal, they are the same. In effect, Decimal128 is a representation of mathematical values having up to 34 significant digits (but without trailing zeroes) and an exponent range of -6143 to +6144. IEEE 754 Decimal128 specifies rules for how the precision of the arguments of an operation carry over to the precision of the result. We do not expose this information. You can think of the Decimal128 values in this package as points on the number line, or mathematical values.

Quick start

import { Decimal128 } from "proposal-decimal";
let a = new Decimal128("0.1");
let b = new Decimal128("0.2");
let c = a.add(b);
console.log(c.toString() === "0.3"); // true

API

Here are the operations that this package supports:

Arithmetic

• absolute value (abs)
• negation (negate)
• addition (add)
• subtraction (subtract)
• multiplication (multiply)
• division (divide)
• remainder (remainder)

Rounding

There is a single round method that takes a rounding mode as an argument. The rounding modes are:

• "ceil"
• "floor"
• "trunc"
• "halfEven" (the default, as with JS Numbers)
• "halfExpand"

Serialization

• toString emitting both decimal and exponential syntax (like Number.prototype.toString)
• toFixed for fixed-point notation (like Number.prototype.toFixed)
• toPrecision for emitting a string with a specified number of significant digits (like Number.prototype.toPrecision)

Conversion

• toNumber to convert to a JavaScript Number
• toBigInt to convert to a JavaScript BigInt (throws for non-integers)

Construction

Decimal128 values can be constructed from:

• a String in decimal notation (e.g., "0.1")
• a String in exponential notation (e.g., "1e-1")
• one of the String values "NaN", "Infinity", "-Infinity"
• a Number (e.g., 0.1)
• a BigInt (e.g., 42n)

Predicates

• isNaN to check for NaN
• isFinite to check for a finite number
• isZero to check for zero
• isNegative to check for a negative number

Comparisons

• equality (equals) to compare for mathematical equality
• difference (notEquals) to compare for mathematical inequality
• less than (lessThan) to compare mathematical order
• less than or equal to (lessThanOrEquals) to compare mathematical order
• greater than (greaterThan) to compare mathematical order
• greater than or equal to (greaterThanOrEquals) to compare mathematical order
• compare (cmp) to compare mathematical order

Why not just less-than and equals? The reason for the proliferation of all these comparisons is due to NaN and -0, which is not 0). Thus, simply negating lessThan does not give you greaterThanOrEquals. Even if you know you're dealing with non-NaN values, you need to watch out for -0.

Installation

After installing this NPM package (npm install proposal-decimal), the dist subdirectory contains a single ESM module, Decimal128.mjs. It exports a single class, Decimal128.

To use this package in Node.js, you can import the module as follows:

import { Decimal128 } from "proposal-decimal";
// your code goes here
// for example:
const x = new Decimal128("0.1");
const y = new Decimal128("0.2");
console.log(x.add(y).toString());

To use this package in a browser, try something like this:

<script type="module">
    import { Decimal128 } from "/path/to/dist/Decimal128.mjs";
    // your code goes here
    // for example:
    const x = new Decimal128("0.1");
    const y = new Decimal128("0.2");
    console.log(x.add(y).toString());
</script>

Implementation

This package is written in TypeScript. Unit tests are in Jest. There are other external dependencies.

Examples

The examples subdirectory contains some example code for your inspection.

Issues

Please file an issue if you find a bug or have a feature request.