== Why is this an issue? Numbers in JavaScript are stored in https://en.wikipedia.org/wiki/Double-precision_floating-point_format[double-precision 64-bit binary format IEEE 754]. Like any other number encoding occupying a finite number of bits, it is unable to represent all numbers. The values are stored using 64 bits in the following form: * 1 bit for the sign (positive or negative) * 11 bits for the exponent (2^n^). -1022 ≤ n ≤ 1023 * 52 bits for the significand (or mantissa) // When images can be added, add image based on to this one //image::IEEE_754_Double_Floating_Point_Format.svg.png[] The actual value of the stored number will be `pass:n[(-1)^sign^ * (1 + significand) * 2 ^exponent^]` Given this structure, there are limits in both *magnitude* and *precision*. Due to the 52 bits used for the significand, any arithmetic in need of more precision than 2^-52^ (provided by `Number.EPSILON`) is subject to rounding. In terms of magnitude, the largest number the 64 bits of the format can store is 2^1024^ - 1 (`Number.MAX_VALUE`). However, because the 52 bits of the significand, only integers between -(2^53^ - 1) (`Number.MIN_SAFE_INTEGER`) and 2^53^ - 1 (`Number.MAX_SAFE_INTEGER`) can be represented exactly and be properly compared. [source,javascript] ---- Number.MAX_SAFE_INTEGER + 1 === Number.MAX_SAFE_INTEGER + 2; // true ---- JavaScript provides the `bigint` primitive to represent values which are too large to be represented by the number primitive. BigInts are created by appending `n` to the end of an integer literal, or by calling the `BigInt()` function (without the new operator), with an integer or a string. [source,javascript] ---- const myBigInt = BigInt(Number.MAX_SAFE_INTEGER); myBigInt + 1n === myBigInt + 2n; // false ---- == How to fix it For large numbers, JavaScript provides the helper function `Number.isSafeInteger()` to test if a number is between the safe limits. When you need to store a large number, use `BigInt`. `bigint` and `number` primitives can be compared between them as usual (e.g. `>`, `==`), but pay attention that arithmetic operations (`+` `pass:[*]` `-` `%` `++**++`) between both types raise an error unless they are converted to the same type. Use the `BigInt` and `Number` functions to convert between both types: [source,javascript] ---- const myNumber = Number(myBigInt); const myBigInt = BigInt(myNumber); ---- Be careful converting values back and forth, however, as the precision of a `bigint` value may be lost when it is coerced to a `number` value. === Code examples ==== Noncompliant code example [source,javascript] ---- const foo = 2312123211345545367 // Noncompliant: will be stored as 2312123211345545000 const bar = BigInt(2312123211345545367); // Noncompliant: parameter is first parsed as an integer and thus rounded ---- ==== Compliant solution [source,javascript] ---- const foo = BigInt('2312123211345545367'); const bar = 2312123211345545367n; ---- When in need of more decimal precision, it is recommended to use a dedicated library to ensure that calculation errors are not introduced by rounding. ==== Noncompliant code example [source,javascript] ---- const baz = 0.123456789123456789 // Noncompliant: will be stored as 0.12345678912345678 ---- ==== Compliant solution [source,javascript] ---- // use a library like decimal.js for storing numbers containing many decimal digits import { Decimal } from 'decimal.js'; const bar = new Decimal('0.123456789123456789'); ---- //=== Pitfalls //=== Going the extra mile == Resources === Documentation * MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number#number_encoding[Number encoding] * MDN web docs - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt[BigInt] * Wikipedia - https://en.wikipedia.org/wiki/Double-precision_floating-point_format[Double-precision floating-point format] * Wikipedia - https://en.wikipedia.org/wiki/IEEE_754[IEEE 754 Standard] //=== Articles & blog posts //=== Conference presentations //=== Standards