big-amount

BigInt-based rational number library focused on accounting

• Stores amount as ratio of two BigInts, allowing precise calculation of arbitrarily large rational numbers
• Optimized for addition of fractions that share same denominator; little overhead on aggregating many cents (1/100)
• Supports rounding and formatting as decimal

import { q, BigAmount } from "big-amount";

let f = q("1/2")           // Same as `BigAmount.create("1/2")`
  .neg()                   // Unary `-`
  .inv()                   // Inverse (`1 / f`)
  .add(q("34.5"))          // `+`
  .sub(q(".67"))           // `-`
  .mul(q(-8n, 9n))         // `*`
  .div(q(10))              // `/`
  .abs()                   // To absolute value
  .reduce();               // To irreducible form

console.log(f.toJSON());   // "1061/375"
console.log(f.toFixed(6)); // "2.829333"

let s = BigAmount.sum([
  "2200811.81",
  "5954398.62",
  "-6217732.25",
  "-9336803.50",
]).toFixed(2, {
  groupSeparator: ",",
  templates: ["${}", "$({})"],
});
console.log(s); // "$(7,399,325.32)"

Note that this library requires ES2020 compatibility (as does BigInt).

Instance Creation

q and BigAmount.create

Use BigAmount.create or the equivalent shortcut q to create an instance.

BigAmount.create(x) creates an instance representing x/1.

q(123n);        // 123/1
q(123);         // 123/1
q("123");       // 123/1
q("123.45");    // 12345/100
q("123.45e-6"); // 12345/100000000

q("123/45");    // 123/45
q("12.3/-4.5"); // 1230/-450

BigAmount.create(x, y) creates an instance representing x/y.

q(123n, 45n);    // 123/45
q(123, 45);      // 123/45

q(123, 45n);     // 123/45
q(123n, "4.5");  // 1230/45

q("1/2", "3/4"); // 4/6
q("1/2", "3.4"); // 10/68

Note that non-integral number values have to be passed as string.

q(123.45);   // ERROR!
q(123 / 45); // ERROR!

BigAmount.fromNumber

Use BigAmount.fromNumber to create an instance from a non-integral number value.

BigAmount.fromNumber(Math.PI); // 3.14159...

Arithmetic Operations

| Methods | Operation | | ------------ | ------------------------------- | | neg() | Negation (unary -) | | add(other) | Addition (this + other) | | sub(other) | Subtraction(this - other) | | mul(other) | Multiplication (this * other) | | div(other) | Division (this / other) | | inv() | Inverse (1 / this) | | abs() | To absolute value | | reduce() | To irreducible form |

reduce() needs to be called explicitly when an irreducible fraction is necessary, because the arithmetic operations do not return the canonical form of a resulting fraction. It is currently guaranteed that add() and sub() keep the denominator unchanged if both operands share the same denominator; in all other cases, the returned object will have implementation-dependent numerator and denominator.

The following methods are also available for convenience and optimization to handle accounting use cases.

| Method | Operation | | ---------------------- | --------------------------------------------------- | | batchAdd(others) | Adds array of BigAmount values at a time | | fixedAdd(other) | Addition that keeps the denominator unchanged | | fixedSub(other) | Subtraction that keeps the denominator unchanged | | fixedMul(other) | Multiplication that keeps the denominator unchanged | | fixedDiv(other) | Division that keeps the denominator unchanged | | quantMul(other, den) | Multiplication that resets the denominator to den | | quantDiv(other, den) | Division that resets the denominator to den |

Rounding

round() rounds a fraction at a decimal place like the round functions of Python and Excel.

q("123.456").round(2);  // 12346/100
q("123.456").round(-1); // 120/1

Use quantize() to convert a fraction to another that has the specified denominator.

q("123/4").quantize(2n); // 62/2

The rounding functions round ties to the nearest even numbers (i.e. bankers' rounding) by default. See RoundingMode for rounding mode options.

Conversion to String

toJSON() generates the preferred form to seriarize a rational number.

JSON.stringify(q("12.345")); // '"12345/1000"'

Use toFixed() to format a fraction as decimal. This method takes some formatting options to customize the output.

let f = BigAmount.create("123456789/10");
f.toFixed(2);                            // "12345678.90"
f.toFixed(2, { decimalSeparator: "," }); // "12345678,90"
f.toFixed(2, { groupSeparator: "," });   // "12,345,678.90"
f.neg().toFixed(2, {
  decimalSeparator: ",",
  groupSeparator: " ",
  templates: ["{} €"],
});                                      // "-12 345 678,90 €"

const opts = { templates: ["${}", "$({})", "-"] };
BigAmount.create("123.45").toFixed(2, opts); // "$123.45"
BigAmount.create("-678.9").toFixed(2, opts); // "$(678.90)"
BigAmount.create("0").toFixed(2, opts);      // "-"

toExponential() returns a string representing a fraction in exponential notation.

q("12345.678").toExponential(6); // "1.234568e+4"

License

Licensed under the Apache License, Version 2.0.

See Also

• Complete API documentation
• Run tests on your browser