Numbers and explicit conversion

1. Conversion and grammar are separate

Number converts a complete numeric string but accepts forms such as whitespace and exponent notation. Number("") is zero. parseInt can accept a numeric prefix and ignore later characters. Choose whether you want a numeric value or an exact text grammar before using either.

console.log(Number(""));
console.log(Number("1e2"));
console.log(Number.isNaN(Number("12pages")));
console.log(Number.parseInt("12pages", 10));

0
100
true
12

2. State a strict count rule

This example accepts only ASCII decimal digits, then checks the numeric range. The negative character test rejects any character outside 0–9, including line separators. It avoids relying on a $ anchor to mean absolute end-of-string. Leading zeroes are accepted deliberately.

function parsePages(raw) defines a named rule: raw receives the supplied text, return gives back the checked number, and throw stops the rule with an error. Lesson 11 develops functions in detail; for now follow those three parts of the complete example.

function parsePages(raw) {
  if (typeof raw !== "string" || raw.length === 0 || /[^0-9]/u.test(raw)) {
    throw new Error("Use decimal digits only");
  }
  const value = Number(raw);
  if (!Number.isSafeInteger(value) || value < 1 || value > 50) {
    throw new Error("Choose 1 to 50 pages");
  }
  return value;
}
console.log(parsePages("12"));

12

3. Approximate numbers have limits

Number uses binary floating-point. Do not assume decimal fractions are exact or that every large integer is representable. Whole cents can be convenient for a small money example but are not a complete accounting policy. Format for display separately from the arithmetic rule.

console.log((0.1 + 0.2).toFixed(2));
console.log(Number.isSafeInteger(9007199254740992));

0.30
false