Check if a string starts with http using Javascript

This should work:

var pattern = /^((http|https|ftp):\/\/)/;

if(!pattern.test(url)) {
    url = "http://" + url;
}

jsFiddle

export const getValidUrl = (url = "") => {
    let newUrl = window.decodeURIComponent(url);
    newUrl = newUrl.trim().replace(/\s/g, "");

    if(/^(:\/\/)/.test(newUrl)){
        return `http${newUrl}`;
    }
    if(!/^(f|ht)tps?:\/\//i.test(newUrl)){
        return `http://${newUrl}`;
    }

    return newUrl;
};

Tests:

expect(getValidUrl('https://www.test.com')).toBe('https://www.test.com');
expect(getValidUrl('http://www.test.com')).toBe('http://www.test.com');
expect(getValidUrl('    http   :    /  /  www.test.com')).toBe('http://www.test.com');
expect(getValidUrl('ftp://www.test.com')).toBe('ftp://www.test.com');
expect(getValidUrl('www.test.com')).toBe('http://www.test.com');
expect(getValidUrl('://www.test.com')).toBe('http://www.test.com');
expect(getValidUrl('http%3A%2F%2Fwww.test.com')).toBe('http://www.test.com');
expect(getValidUrl('www    .  test.com')).toBe('http://www.test.com');