Decimal: JavaScript's future numeric type

## Decimal: The Need for Precision in JavaScript and the Future with TC39

At its core, computing deals with numbers. However, the way we represent and manipulate these numbers can have profound implications, especially in domains where precision is crucial, such as in financial, scientific, or engineering applications. JavaScript has historically faced challenges in this area with its native floating-point number representation, opening the door for the need for more precise data types like Decimal.

The Problem with Floating-Point in JavaScript

JavaScript utilizes the IEEE 754 standard for representing double-precision (64-bit) floating-point numbers. While efficient for most use cases, this representation can lead to subtle inaccuracies in certain operations. The fundamental reason lies in how decimal numbers are converted to binary. Many decimal numbers that appear simple in base 10 (like 0.1 or 0.2) do not have an exact finite binary representation. This results in small discrepancies that can accumulate and cause unexpected errors.

Consider a classic example:

// Example in pure JavaScript
let result = 0.1 + 0.2;
console.log(result); // Output: 0.30000000000000004

This output, 0.30000000000000004, instead of the expected 0.3, demonstrates the inherent problem with floating-point representation. In financial applications, where every cent matters, this type of inaccuracy is unacceptable.

The Rise of Decimal and TC39 Status

To mitigate these issues, Decimal libraries have emerged as popular solutions. These libraries implement decimal numbers using internal representations that ensure exact precision, typically by storing numbers as strings or arrays of digits.

However, the need for a native, standardized solution has led to proposals for including a Decimal type in the ECMAScript specification (the basis of JavaScript). The proposal for a Decimal type has reached Stage 2 (Working Draft) of the standardization process by TC39 (Technical Committee 39), the committee responsible for the evolution of ECMAScript. This means the proposal is under active development and review, with the goal of becoming a standard language feature.

The current status of the proposal indicates a promising path toward the native addition of a Decimal type, which would simplify development and eliminate reliance on external libraries for many use cases.

Comparing Decimal with BigInt

It's important not to confuse Decimal with BigInt. While both address limitations of JavaScript's primitive numbers, they serve different purposes:

• Decimal: Focuses on exact precision for numbers with decimal places, resolving floating-point issues in operations involving fractions and monetary calculations. It deals with the precise representation of numbers that may not have an exact finite binary representation.

• BigInt: Introduced in the ECMAScript 2020 specification, BigInt is a primitive numeric type that can represent arbitrarily large integers, overcoming the limit of Number.MAX_SAFE_INTEGER. It is ideal for working with integers that exceed the capacity of standard floating-point numbers.

Let's illustrate with TypeScript examples, demonstrating best practices:

// Example of using a Decimal library (simulated for educational purposes)
// In a real scenario, you would use a library like 'decimal.js' or 'big.js'

/**
 * Represents a decimal number with exact precision.
 * In a real implementation, this would be a class encapsulating the logic.
 */
class PreciseDecimal {
    private value: string; // Stores the number as a string for precision

    constructor(num: number | string) {
        // In a real implementation, there would be robust validation and conversion.
        this.value = String(num);
    }

    add(other: PreciseDecimal): PreciseDecimal {
        // Complex decimal addition logic here.
        // For this example, we just simulate the correct result.
        const num1 = parseFloat(this.value);
        const num2 = parseFloat(other.value);
        // Note that this simulation still uses parseFloat internally for demonstration;
        // a real implementation would not do this for the core operation.
        return new PreciseDecimal((num1 + num2).toFixed(10)); // toFixed to simulate precise result
    }

    // Other methods like subtract, multiply, divide, etc.

    toString(): string {
        return this.value;
    }
}

// Using the simulated PreciseDecimal class
const amount1 = new PreciseDecimal(0.1);
const amount2 = new PreciseDecimal(0.2);
const preciseSum = amount1.add(amount2);

console.log(`Precise decimal sum: ${preciseSum.toString()}`); // Expected output: 0.3 (or similar, depending on configured precision)

// Example with BigInt
// Working with very large integers
const veryLargeNumber1: bigint = BigInt(\"900719925474099100\"); // Exceeding MAX_SAFE_INTEGER
const veryLargeNumber2: bigint = BigInt(\"100");

const bigIntSum: bigint = veryLargeNumber1 + veryLargeNumber2;

console.log(`BigInt sum: ${bigIntSum}`); // Output: 900719925474099200

Conclusion

Precision is a fundamental pillar in many applications. As JavaScript evolves, the need to handle decimal numbers accurately becomes increasingly pressing. The proposal for a Decimal type in TC39 represents a significant step toward a more robust and reliable JavaScript ecosystem.

Understanding the difference between Decimal and BigInt is crucial for choosing the right tool for the job: Decimal for precision with decimal places and BigInt for large-magnitude integers. As the standard evolves, we can expect the development of high-precision applications in JavaScript to become more straightforward and less prone to subtle errors.