net.ipv4

/**
 * The address as an unsigned 32-bit number.
 *
 * Only the strict dotted quad is accepted. The C library's inet_aton also
 * reads "010.1.1.1" as octal 8.1.1.1, "0x7f.1" as hex and "127.1" as
 * 127.0.0.1; a validator that agrees with one reader and a socket library that
 * agrees with the other is how an allow-list gets bypassed, so all of those
 * are refused.
 */
export function parseIpv4(text: string): number {
  const value = ipv4Value(text);
  if (value === null) throw new Error(`"${String(text)}" is not a dotted-quad IPv4 address`);
  return value;
}

// Exported for isIpv4 and for capabilities that parse IPv4 text without
// wanting an error; not part of the group's contract.

/** The address as a number, or null when the text is not a strict dotted quad. */
export function ipv4Value(text: unknown): number | null {
  if (typeof text !== "string") return null;
  const parts = text.split(".");
  if (parts.length !== 4) return null;
  let value = 0;
  for (const part of parts) {
    if (part.length < 1 || part.length > 3) return null;
    for (const ch of part) if (ch < "0" || ch > "9") return null;
    // A leading zero is where octal readers and decimal readers part company.
    if (part.length > 1 && part[0] === "0") return null;
    const octet = Number(part);
    if (octet > 255) return null;
    value = value * 256 + octet;
  }
  return value;
}

/** Dotted-quad text for an unsigned 32-bit address: the inverse of parseIpv4. */
export function formatIpv4(value: number): string {
  if (typeof value !== "number" || !Number.isInteger(value) || value < 0 || value > 4294967295) {
    throw new Error(`IPv4 value must be an integer from 0 to 4294967295, received ${value}`);
  }
  // Arithmetic rather than >>> and &, which work on signed 32-bit numbers.
  return [
    Math.floor(value / 16777216) % 256,
    Math.floor(value / 65536) % 256,
    Math.floor(value / 256) % 256,
    value % 256,
  ].join(".");
}

Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.

import { ipv4Value } from "./net_ipv4_parse_ipv4.ts";  ← parseIpv4, another function of this group · built into the same file, even by a slim install

/** True exactly when parseIpv4 would accept the text; never throws. */
export function isIpv4(text: string): boolean {
  return ipv4Value(text) !== null;
}