Dates and numbers

In plain English

A Date object represents a moment in time, and its timestamp is the number of milliseconds since 1 January 1970, which is how computers compare and sort dates. You create a date with new Date() and read its parts, like the year or month. Intl.NumberFormat formats numbers for a locale by adding commas, currency symbols and the right number of decimals. JavaScript numbers use floating point, so decimals like 0.1 cannot be stored exactly, which is why 0.1 + 0.2 is not quite 0.3; round money at the end with toFixed or work in whole cents.

Step by step

1. 1Create a Date and read its year and month.
2. 2Subtract two dates to get the difference in milliseconds, then convert to days.
3. 3Format a number as currency with Intl.NumberFormat.
4. 4Check that 0.1 + 0.2 is not exactly 0.3, then round it with toFixed.

const now = new Date();
console.log(now.getFullYear());

const start = new Date("2024-01-01");
const end = new Date("2024-01-08");
const diffMs = end - start;
const diffDays = diffMs / (1000 * 60 * 60 * 24);
console.log(diffDays);

const money = new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD"
});
console.log(money.format(1234.5));

console.log(0.1 + 0.2);
const price = (0.1 + 0.2).toFixed(2);
console.log(price);

Common mistakes

• Using getMonth() and forgetting it is zero-based, so January is 0.
• Expecting 0.1 + 0.2 to equal exactly 0.3 in JavaScript.
• Comparing dates as strings instead of using their timestamps.
• Rounding money with Math.round on a decimal instead of working in whole cents.