Coding challenge to convert a 6-digit hex color code to its nearest "web safe" equivalent

color_safe.js

/**
 * Given an input of a 6-character hex color code, we need to output a "web-safe"
 * representation of that same code.
 */

'use strict';

let assert = (one, two) => {
    if (one === two) {
        console.info(`Success! ${one} is equal to ${two}!`);
    } else {
        console.error(`Failbomb! ${one} is _NOT_ equal to ${two}!`);
    }
};

const convert = hex_input => {
    let valid_input = validate(hex_input);

    let rgb_values = hex_to_decimal(valid_input);
    let rounded_rgb_values = [];
    let rounded_hex_rgb_values = [];

    rgb_values.forEach(i => {
        // How to round to multiple of 51?
        let safe_value = closest_multiple_of_51(i);
        rounded_rgb_values.push(safe_value);
    });

    // convert back to hex
    rounded_rgb_values.forEach(i => {
        rounded_hex_rgb_values.push(i.toString(16));
    });

    return `#${rounded_hex_rgb_values.join('')}`;
};

const validate = string => {
    // TODO: normalize valid 3 character color strings to 6 characters; e.g. #333 => #333333
    // TODO: actual work here, ensure a 6-character hex string
    return string;
};

const hex_to_decimal = hex_input => {
    // hex_input: #000000
    let red_hex = hex_input.substr(1, 2);
    let green_hex = hex_input.substr(3, 2);
    let blue_hex = hex_input.substr(5, 2);

    let red_decimal = parseInt(red_hex, 16);
    let green_decimal = parseInt(green_hex, 16);
    let blue_decimal = parseInt(blue_hex, 16);

    return [red_decimal, green_decimal, blue_decimal];
};

const closest_multiple_of_51 = number => {
    let remainder = number % 51;

    if (remainder === 0) {
        return number;
    }

    if (remainder > 51 / 2) {
        number += (51 - remainder);
    } else {
        number -= remainder;
    }

    return number;
};

assert(closest_multiple_of_51(0), 0);
assert(closest_multiple_of_51(255), 255);
assert(closest_multiple_of_51(45), 51);
assert(closest_multiple_of_51(65), 51);
assert(closest_multiple_of_51(111), 102);
assert(closest_multiple_of_51(160), 153);
assert(closest_multiple_of_51(90), 102);

assert(closest_multiple_of_51(24), 0);
assert(closest_multiple_of_51(25), 0);
assert(closest_multiple_of_51(26), 51);

let results = hex_to_decimal('#000033');
assert(results[0], 0);
assert(results[1], 0);
assert(results[2], 51);

results = hex_to_decimal('#ae3312');
assert(results[0], 174);
assert(results[1], 51);
assert(results[2], 18);

assert(convert('#000044'), '#000033');
assert(convert('#ae3312'), '#993300');