TryGhost/Ghost

import isEmail from 'validator/es/lib/isEmail';

export const formatUrl = (value: string, baseUrl?: string, nullable?: boolean) => {
    if (nullable && !value) {
        return {save: null, display: ''};
    }

    let url = value.trim();

    if (!url) {
        if (baseUrl) {
            return {save: '/', display: baseUrl};
        }
        return {save: '', display: ''};
    }

    // if we have an email address, add the mailto:
    if (isEmail(url)) {
        return {save: `mailto:${url}`, display: `mailto:${url}`};
    }

    const isAnchorLink = url.match(/^#/);
    if (isAnchorLink) {
        return {save: url, display: url};
    }

    const isProtocolRelative = url.match(/^(\/\/)/);
    if (isProtocolRelative) {
        return {save: url, display: url};
    }

    if (!baseUrl) {
        // Absolute URL with no base URL
        if (!url.startsWith('http')) {
            url = `https://${url}`;
        }
    }

    // If it doesn't look like a URL, leave it as is rather than assuming it's a pathname etc
    if (!url.match(/^[a-zA-Z0-9-]+:/) && !url.match(/^(\/|\?)/)) {
        return {save: url, display: url};
    }

    let parsedUrl: URL;

    try {
        parsedUrl = new URL(url, baseUrl);
    } catch (e) {
        return {save: url, display: url};
    }

    if (!baseUrl) {
        return {save: parsedUrl.toString(), display: parsedUrl.toString()};
    }
    const parsedBaseUrl = new URL(baseUrl);

    let isRelativeToBasePath = parsedUrl.pathname && parsedUrl.pathname.indexOf(parsedBaseUrl.pathname) === 0;

    // if our path is only missing a trailing / mark it as relative
    if (`${parsedUrl.pathname}/` === parsedBaseUrl.pathname) {
        isRelativeToBasePath = true;
    }

    const isOnSameHost = parsedUrl.host === parsedBaseUrl.host;

    // if relative to baseUrl, remove the base url before sending to action
    if (isOnSameHost && isRelativeToBasePath) {
        url = url.replace(/^[a-zA-Z0-9-]+:/, '');
        url = url.replace(/^\/\//, '');
        url = url.replace(parsedBaseUrl.host, '');
        url = url.replace(parsedBaseUrl.pathname, '');

        if (!url.match(/^\//)) {
            url = `/${url}`;
        }
    }

    if (!url.match(/\/$/) && !url.match(/[.#?]/)) {
        url = `${url}/`;
    }

    // we update with the relative URL but then transform it back to absolute
    // for the input value. This avoids problems where the underlying relative
    // value hasn't changed even though the input value has
    return {save: url, display: displayFromBase(url, baseUrl)};
};

const displayFromBase = (url: string, baseUrl: string) => {
    // Ensure base url has a trailing slash
    if (!baseUrl.endsWith('/')) {
        baseUrl += '/';
    }

    // Remove leading slash from url
    if (url.startsWith('/')) {
        url = url.substring(1);
    }

    return new URL(url, baseUrl).toString();
};