Replacing // With Math.floor()

Replacing // With Math.floor()

In this lesson, you ported the batch planner’s full-batches line, the one that went quietly wrong in the course introduction, and found out why: JavaScript has no floor division operator.

• In Python, green_kg // DRUM_KG divides and rounds down. On a float it gives 3.0, so the script wraps it in int() to get 3.
• The port lives in batch-plan.js, in the folder batch-plan/, whose package.json holds { "type": "module" }.
• JavaScript has one number type, so the port has no int to convert to, and drops the int() call.
• In JavaScript, // starts a comment, as # does in Python, and the comment runs to the end of the line. So const fullBatches = greenKg // DRUM_KG; is really const fullBatches = greenKg, and it runs with no error.
• Inside a call’s parentheses, the comment swallows the closing one. Then the file doesn’t run at all: JavaScript reports SyntaxError: missing ) after argument list before running a single line.
• Division in JavaScript is always /, which keeps the fraction. Math.floor() rounds the result down, as Python’s // does, and shares its name with Python’s math.floor(). Math holds JavaScript’s math functions, like Python’s math module, and it gets a lesson of its own soon.
• Python’s int() has two jobs. In the batches line it only turned 3.0 into 3, which JavaScript doesn’t need. Given decimals, it chops them off, and that’s Math.trunc(): Math.trunc(greenKg / DRUM_KG) is 3, like int(green_kg / DRUM_KG). For the planner’s weights, which are never negative, it gives the same 3 as Math.floor().
• Watch out: for values that can go below zero, such as a time offset, the two part ways. Math.floor() goes down the number line and Math.trunc() goes toward zero.
• So Math.floor(-7 / 2) is -4, like Python’s -7 // 2, while Math.trunc(-7 / 2) is -3, like int(-7 / 2).
• When you port //, say which rounding you mean, above all when values can go negative: Math.floor() rounds down, as // does, and Math.trunc() chops the decimals, as int() does.

Here’s the port as first carried over from Python, with the // kept:

const WEIGHT_LOSS = 0.15;
const DRUM_KG = 12;

const orderKg = 37.5;
const greenKg = orderKg / (1 - WEIGHT_LOSS);
const fullBatches = greenKg // DRUM_KG;
console.log(`Batches: ${fullBatches} full`);