Numbers, Math, and Dates

Numbers, Math, and Dates | JavaScript Course | SovranCode

COMPLETE LESSON · ABOUT 30 MIN OF ACTIVE PRACTICE

Learn Numbers, Math, and Dates step by step

Follow the lesson in order: understand the model, trace worked examples, run a controlled experiment, debug a faulty assumption, build independently, and verify your reasoning.

LESSON MAP

What you will understand and apply

Reject non-finite and out-of-range input
Keep money in integer minor units
Use Math methods only after defining rounding rules
Create a cart calculation in integer cents with validation for price and quantity.

DEEP EXPLANATION

Build the complete mental model

Build the mental model

Number converts an entire input, while parseInt and parseFloat accept a numeric prefix. That difference matters for values such as '12px'. Number.isFinite rejects NaN and infinities without coercing strings. Number.isInteger confirms whole numbers but does not enforce a useful range. Conversion, type validation, and domain validation are three separate decisions. Before writing more syntax, name the values involved in Numbers, Math, and Dates, who creates them, and which result or effect the program promises. Trace the smallest example by hand and identify any browser or runtime API separately from the language feature. A useful mental model should predict a changed input, not merely describe the demonstration after it runs.

Use the API deliberately

Binary floating point cannot represent every decimal fraction exactly, so 0.1 + 0.2 is not exactly 0.3. Financial code commonly stores integer minor units such as cents, applies explicit rounding at agreed boundaries, and formats only for display. Math.round, floor, ceil, trunc, min, max, and random have different contracts; name the rule instead of choosing whichever produces a convenient sample. During implementation, keep the Number.isFinite rule in a focused function or event boundary with explicit inputs. Choose names that reveal the domain, keep calculation separate from document, console, storage, or network effects, and prefer the narrowest API that expresses the problem. Compare this design with one reasonable alternative and explain why the selected construct makes ownership or control flow clearer.

Handle boundaries and tradeoffs

Date represents a timestamp in milliseconds and can display it through local or UTC methods. Date-only strings, time zones, daylight changes, and invalid dates create subtle boundaries. Store an unambiguous instant when that is the domain, format through Intl.DateTimeFormat for the user, and avoid manual month arithmetic when a dedicated date library or Temporal is required. Verification for Numbers, Math, and Dates must cover an ordinary case, an empty or missing case, invalid input, and the most important threshold. Predict each result before running it, then locate the first value that violates the contract when reality differs. Record the corrected case as a repeatable check and explain which product behavior should occur instead of allowing undefined, NaN, stale data, or silent failure to spread.

WORKED EXAMPLES

Read the decision before the syntax

Example 1: Primary worked example

Trace this Numbers, Math, and Dates example from validated input to returned value or visible effect before editing it.

function lineTotalCents(priceCents, quantity) {
  if (!Number.isInteger(priceCents) || priceCents < 0) throw new RangeError("Invalid price");
  if (!Number.isInteger(quantity) || quantity < 1) throw new RangeError("Invalid quantity");
  return priceCents * quantity;
}
console.log({ totalCents: lineTotalCents(1295, 3) });

Example 2: Second worked example

This variation changes the shape of the problem so you can compare the underlying Number.isFinite contract instead of copying one solution.

const formatter = new Intl.NumberFormat("en", { style: "currency", currency: "USD" });
function formatCents(cents) {
  if (!Number.isSafeInteger(cents)) throw new TypeError("Safe integer cents required");
  return formatter.format(cents / 100);
}
console.log(formatCents(3895));

How to trace this lesson

Write down the exact inputs used by Numbers, Math, and Dates.
Predict each returned value and observable effect before running the example.
Mark the first boundary where invalid or missing data could enter.
Verify the ordinary, empty, invalid, and threshold cases independently.

Common mistakes and corrections

Comparing decimal calculations for exact equality can expose binary floating-point representation rather than the intended business rule.: Make the Number.isFinite rule explicit and cover it with a focused example.
Changing several parts of the example before explaining the original result.: Make one change at a time and compare it with a written prediction.
Testing only the demonstration's happy path.: Add missing, invalid, empty, and boundary inputs that exercise the stated contract.

PRACTICE LADDER

Move from recognition to independent work

1
Reject non-finite and out-of-range input
2
Keep money in integer minor units
3
Use Math methods only after defining rounding rules
4
Create a cart calculation in integer cents with validation for price and quantity.

Complete these in order. The final step should work without copying the lesson example; use the runner and lab below to test it.

Core rule

Applied boundary

RUNNABLE EXAMPLE

Change one assumption at a time

Predict the output, run the example, then change one input and one boundary in the Numbers, Math, and Dates example. Explain every changed result before continuing.

function lineTotalCents(priceCents, quantity) {
  if (!Number.isInteger(priceCents) || priceCents < 0) throw new RangeError("Invalid price");
  if (!Number.isInteger(quantity) || quantity < 1) throw new RangeError("Invalid quantity");
  return priceCents * quantity;
}
console.log({ totalCents: lineTotalCents(1295, 3) });

function lineTotalCents(priceCents, quantity) {
  if (!Number.isInteger(priceCents) || priceCents < 0) throw new RangeError("Invalid price");
  if (!Number.isInteger(quantity) || quantity < 1) throw new RangeError("Invalid quantity");
  return priceCents * quantity;
}
console.log({ totalCents: lineTotalCents(1295, 3) });

Press Run to execute your code.

DEBUGGING CHALLENGE

Locate the first incorrect value

Predict the failure and write your repair before revealing the explanation.

const total = "12.50" + 2;

Reveal repair and explanation

const price = Number("12.50");
if (!Number.isFinite(price)) throw new TypeError("Invalid price");
const total = price + 2;

Convert and validate the string before arithmetic so plus performs numeric addition.

INDEPENDENT LAB

Build without copying the example

Create a cart calculation in integer cents with validation for price and quantity.

// Define small functions before composing the complete solution.
// Create a cart calculation in integer cents with validation for price and quantity.

// Define small functions before composing the complete solution.
// Create a cart calculation in integer cents with validation for price and quantity.

Press Run to execute your code.

Self-review checklist Reject non-finite and out-of-range input Keep money in integer minor units Use Math methods only after defining rounding rulesWhat did you predict, observe, and change?

KNOWLEDGE CHECK

Explain the rule, not only the syntax

1. What representation avoids routine decimal rounding errors for money? Integer minor units Formatted currency strings Random rounded floats

2. Which evidence best demonstrates mastery of Numbers, Math, and Dates? A prediction, boundary tests, and an explained result A copied snippet with no changes One successful happy-path run

Loading saved work…

Code and answers save automatically. Guest work stays on this device; signed-in work syncs to your account. The runner is isolated, blocks network access, and stops long-running console programs.

1. JavaScript Foundations

2. Functions and Collections

3. Objects and Structured Data

4. Browser Interfaces

5. Async and Reliable Applications

6. Application Engineering