Rounding With Math.round()

Rounding With Math.round()

In this lesson, you ported the batch planner’s bags and cost lines, and found where JavaScript’s rounding parts ways with Python’s.

• JavaScript has no math module to import. Its math functions live on Math, with a capital M, a built-in global object that’s always there. Watch out: JavaScript’s form of the import, import math from "math", stops the file with ERR_MODULE_NOT_FOUND before any line runs.
• math.ceil() becomes Math.ceil(), which rounds up as Python’s does, so the 37.5 kg order needs 150 bags, though it gives back a plain number, not an int. Math also holds Math.sqrt() and Math.PI (Python’s math.pi), and Math.random() for random numbers.
• Python’s built-ins round(), abs(), max() and min() move onto Math too. Watch out: abs() or round() on its own is a ReferenceError.
• Math.abs(-75) is 75, and Math.max(12, 8, 17.5) and Math.min(12, 8, 17.5) are 17.5 and 8. They take separate values; finding the largest in an array comes later.
• Math.round() always rounds to a whole number: unlike Python’s round(), it has no place for a number of decimals. Where Python finds no place for an argument, it raises a TypeError, but JavaScript doesn’t check how many arguments you pass, and Math.round() reads only the first. So Math.round(greenKg * GREEN_PRICE_PER_KG, 2) gives 326, and the planner prints Green coffee: €326 where Python prints Green coffee: €326.47.
• To round to cents, scale up, round, and scale back: Math.round(rawCost * 100) / 100. Times 100, the cost is 32647.058823529413 cents, Math.round() makes that 32647, and dividing by 100 gives 326.47.
• That rounds for further calculation. Showing a price with exactly two decimals is a separate job, for a later lesson.
• Watch out for exact halves: Math.round() rounds a half up, toward positive infinity, so Math.round(2.5) is 3 where Python’s round(2.5) is 2. Below zero, Math.round(-2.5) is -2; Python’s round(-2.5) is -2 too, but only because -2 is even. On the cents line, an exact half-cent splits the same way: 0.125 rounds to 0.13 in JavaScript and 0.12 in Python.

Here are just the two new lines and the settings they use; the batch lines stay where they are:

const WEIGHT_LOSS = 0.15;
const BAG_G = 250;
const GREEN_PRICE_PER_KG = 7.4;

const orderKg = 37.5;
const greenKg = orderKg / (1 - WEIGHT_LOSS);
const bags = Math.ceil((orderKg * 1000) / BAG_G);
const rawCost = greenKg * GREEN_PRICE_PER_KG;
const cost = Math.round(rawCost * 100) / 100;
console.log(`Bags: ${bags}`);
console.log(`Green coffee: €${cost}`);