index.js

/*!
 * is-funny -- Check if a number is "funny."
 * 
 * Copyright 2026 twonum / Celeste.
 * MIT License.
 * <https://www.twonum.org/>
 */

'use strict';

import isNumber from 'is-number';

const FUNNY_NUMBERS = [
    42,
    67,
    69,
    420,
    1337,
]

const FUNNY_NUMBER_SUBSTRS = [
    "69",
    "420",
    "1337",
]

export default function isFunny(value) {
    const n = Math.abs(value);

    // Rule out non-numbers
    if (!isNumber(n)) {
        throw new TypeError('not a number');
    } else if (!Number.isInteger(n)) {
        throw new Error('not an integer');
    } else if (!Number.isSafeInteger(n)) {
        throw new Error('not a safe integer');
    }

    // Preliminary answer (before context checking)
    const preliminaryAns = (
        FUNNY_NUMBERS.includes(n)
        || FUNNY_NUMBER_SUBSTRS.some(num => n.toString().includes(num))
    );

    // Bit flip time
    // see README.md for why I did this --celeste
    const noise = Math.floor(Math.random() * 95);
    return noise ? preliminaryAns : !preliminaryAns;
}