SafeMath Class. How to create chainable calculations?

Question

I'm having a hard time finding the keywords to search for this online.

I've created a class with safe math functions. Each function takes 2 arguments and after being evaluated by an assertion, it returns the result.

Example:

class SafeMath {

  static add(x: number, y: number) {
    let z: number = x + y;
    assert(z >= x, 'ds-math-add-overflow');
    return z;
  }

  static sub(x: number, y: number) {
    let z: number = x - y;
    assert(z <= x, 'ds-math-sub-underflow');
    return z;
  }

  static mul(x: number, y: number) {
    let z: number = x * y;
    assert(y == 0 || z / y == x, 'ds-math-mul-overflow');
    return z;
  }

  static div(x: number, y: number) {
    let z: number = x / y;
    assert(x > 0 || y > 0, 'ds-math-div-by-zero');
    return z;
  }

}

console.log(SafeMath.add(2,2)); // 4
console.log(SafeMath.sub(2,2)); // 0
console.log(SafeMath.mul(2,2)); // 4
console.log(SafeMath.div(2,2)); // 1

My goal was to have these functions work like this, for example:

let balance0: number = 1;
let balance1: number = 1;

let amount0In: number = 10;
let amount1In: number = 10;

let balance0Adjusted: number = balance0.mul(1000).sub(amount0In.mul(3));
let balance1Adjusted: number = balance1.mul(1000).sub(amount1In.mul(3));

...the functions would take in y and use the previous number as x.

Answer 1

You can make some wrapper for that:

if (!Number.prototype.mul)  // check that the mul method does not already exist 
  {
  Number.prototype.mul = function(n){ return this * n }
  }
  
if (!Number.prototype.add)
  {
  Number.prototype.add = function(n){ return this + n }
  }
  
  
let val = 5
let doubleValPlus500 = val.mul(2).add(500)

console.log( doubleValPlus500 )

Answer 2

You can modify Number.prototype to add functions so that you can chain these operations. Doing so using string property keys is generally considered a bad practice (see Why is extending native objects a bad practice?). You can use unique symbol property keys instead of string property keys to avoid name conflicts, etc.

Here is an example module which safely "extends" Number.prototype with a multiplication function using a unique symbol and adds to the TypeScript Number interface the new function signature:

mul.ts

const mul = Symbol("multiply");

function value(this: number, n: number) {
  return this * n;
}

declare global {
  interface Number {
    [mul]: typeof value;
  }
}

Object.defineProperty(Number.prototype, mul, { value });

export default mul;

After defining a module like the above for subtraction, addition, division, etc. you can then import the modules and use their exported unique symbols to chain operations:

import mul from "./mul.ts";
import sub from "./sub.ts";

const balance = 1;
const amountIn = 10;
const balanceAdjusted = balance[mul](1000)[sub](amountIn[mul](3));
console.log(balanceAdjusted);

970

A nicety of making these math operations chainable is that you can combine them with the optional chaining operator whenever you're dealing with nullish values which can come in handy sometimes.

---

The same can be done without using symbols but it isn't safe against future versions of JavaScript that might define their own Number methods for mul, etc:

mul.ts

function value(this: number, n: number) {
  return this * n;
}

declare global {
  interface Number {
    mul: typeof value;
  }
}

Object.defineProperty(Number.prototype, "mul", { value });

export {}; // you have to import or export something to make it a module

import "./mul.ts";
import "./sub.ts";

const balance = 1;
const amountIn = 10;
const balanceAdjusted = balance.mul(1000).sub(amountIn.mul(3));
console.log(balanceAdjusted);

970

---

Importing all of these modules individually may not be very convenient so you can also make a single module to combine all the others:

math.ts

export { default as mul } from "./mul.ts";
export { default as sub } from "./sub.ts";
/* and so forth */

Then you can import it and select the ones you want to use:

import { mul, sub } from "./math.ts";

const balance = 1;
const amountIn = 10;
const balanceAdjusted = balance[mul](1000)[sub](amountIn[mul](3));
console.log(balanceAdjusted);

Answer 3

Number.prototype based example

import { assert } from "https://deno.land/std@0.102.0/testing/asserts.ts";

declare global {

  /*
    Augument global Number.prototype with the following custom functions

    Warning - While this may look like a clean approach, it is considered 
    unsafe due to javascript possibly choosing to natively implement these
    exact function names in the near future. To avoid this, choose unique
    function names.

  */
  interface Number {
    add(n: number): number;
    sub(n: number): number;
    mul(n: number): number;
    div(n: number): number;
    pow(n: number): number;
    sqrt(): number;
    print(): number;
  }
}

Number.prototype.add = function(this:number, n:number) {

  assert((this + n) >= this, 'ds-math-add-overflow');
  return this + n;
}

Number.prototype.sub = function(this:number, n:number) {

  assert((this - n) <= this, 'ds-math-sub-underflow');
  return this - n;
}

Number.prototype.mul = function(this:number, n:number) {

  assert(n == 0 || (this * n) / n == this, 'ds-math-mul-overflow');
  return this * n;
}

Number.prototype.div = function(this:number, n:number) {

  assert(this > 0 || n > 0, 'ds-math-div-by-zero');
  return this / n;
}

Number.prototype.pow = function(this:number, n:number) {

  assert(this > 0 && n >= 2, 'ds-math-exp-to-zero');
  return this ** n;
}

// babylonian method (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Babylonian_method)
Number.prototype.sqrt = function(this:number) {

  assert(this > 0, 'ds-math-sqrt-of-zero');

  let x: number = 0;
  let y: number = this;
  let z: number = 0;

  if (y > 3) {
    z = y;
    x = y / 2 + 1;
    while (x < z) {
      z = x;
      x = (y / x + x) / 2;
    }
  } else if (y != 0) {
    z = 1;
  }
  return z;
}

Number.prototype.print = function(this:number) {

  console.log('=', this);
  return this;
}

// Here it is in action:

let balance = 0;

balance.add(10).print().sub(1).print().mul(2).print().div(3).print().pow(4).print().sqrt().print();

Output:

= 10
= 9
= 18
= 6
= 1296
= 36

Class based example

import { assert } from "https://deno.land/std@0.102.0/testing/asserts.ts";

class SafeMath {
  private n: number;

  constructor(start: number = 0) {
    this.n = start;
  }

  public add(y: number) {

    assert(this.n + y >= this.n, 'ds-math-add-overflow');

    let z: number = this.n + y;

    this.n = this.n + y;
    return this;
  }

  public sub(y: number) {

    assert(this.n - y <= this.n, 'ds-math-sub-underflow');

    let z: number = this.n - y;

    this.n = this.n - y;
    return this;
  }

  public mul(y: number) {

    assert(y == 0 || (this.n * y) / y == this.n, 'ds-math-mul-overflow');

    let z: number = this.n * y;

    this.n = this.n * y;
    return this;
  }

  public div(y: number) {

    assert(this.n > 0 || y > 0, 'ds-math-div-by-zero');

    let z: number = this.n / y;

    this.n = this.n / y;
    return this;
  }

  public pow(y: number) {

    assert(this.n > 0 && y >= 2, 'ds-math-exp-to-zero');

    let z: number = this.n ** y;
    this.n = this.n ** y;
    return this;
  }

  // babylonian method (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Babylonian_method)
  public sqrt() {

    assert(this.n > 0, 'ds-math-sqrt-of-zero');

    let x: number = 0;
    let y: number = this.n;
    let z: number = 0;

    if (y > 3) {
      z = y;
      this.n = z;
      x = y / 2 + 1;
      while (x < z) {
        z = x;
        this.n = z
        x = (y / x + x) / 2;
      }
    } else if (y != 0) {
      z = 1;
      this.n = z
    }
    return this;
  }

  public print() {
    console.log('=', this.n);
    return this;
  }

}

// Here it is in action:

new SafeMath(0).add(10).print().sub(1).print().mul(2).print().div(3).print().pow(4).print().sqrt().print();

Output:

= 10
= 9
= 18
= 6
= 1296
= 36