Edit in JSFiddle

/*
string.allIndexOf(searchstring, ignoreCase)
  string [String] - the string to search within for the searchstring
  searchstring [String] - the desired string with which to find starting indexes
  ignoreCase [Boolean] - set to true to make both the string and searchstring case insensitive
*/
(function() {
    String.prototype.allIndexOf = function(string, ignoreCase) {
        if (this === null) { return [-1]; }
        var t = (ignoreCase) ? this.toLowerCase() : this,
            s = (ignoreCase) ? string.toString().toLowerCase() : string.toString(),
            i = t.indexOf(s),
            len = this.length,
            n, indx = 0,
            result = [];
        if (len === 0 || i === -1) { return [i]; } // "".indexOf("") is 0
        for (n = 0; n <= len; n++) {
            i = t.indexOf(s, indx);
            if (i !== -1) {
                indx = i + 1;
                result.push(i);
            } else {
                return result;
            }
        }
        return result;
    };
})();












// *** Demo code ***
$(function() {
    var i, r = $('#results'),
        p = false,
        // make it pretty, also to allow adding more examples ;)
        // w = string (no quotes means it's a variable),
        // s = search string, i = allIndexOf, c = ignore case, n = note
        show = function(w, s, i, c, n) {
            var st = (w.indexOf('"') !== -1 ? '<span class="str">' + w + '</span>' : w) + '.<span class="fn">allIndexOf(</span>' + (typeof s === 'number' ? '<span class="num">' + s : '<span class="str">&quot;' + s + '&quot') + '</span>' + (c ? ', <span class="bool">true</span>' : '') + '<span class="fn">)</span>; <span class="cmt">// result [ ' + i.toString() + ' ]' + (n || '') + '</span><br>';
            r[p ? 'prepend' : 'append'](st);
        },

        s = "The rain in Spain stays mainly in the plain",
        t = "Sally: No1 likes my shoes?... Johnny: So, does 'No1' mean noone or number one? Because I can test the latter!",

        i = "".allIndexOf('x'); // result [ -1 ]
    show('""', 'x', i);

    i = "".allIndexOf(""); // result [ 0 ]
    show('""', '', i, false, ' - expected; "".indexOf("") is 0');

    i = "abcd".allIndexOf(""); // result [ 0 ]
    show('"abcd"', '', i, false, ' - expected; "abcd".indexOf("") is 0 & "abcd".lastIndexOf("") is 4');

    // add variable to code block
    r.append('<br>var s = <span class="str">"' + s + '"</span>;<br>');

    i = s.allIndexOf('z'); // result [ -1 ]
    // 's' = string name being used, 'z' = search, i = allIndexOf result
    show('s', 'z', i);

    i = s.allIndexOf('ain'); // result [5, 14, 25, 40]
    show('s', 'ain', i);

    i = s.allIndexOf('the'); // result [ 34 ]
    show('s', 'the', i);

    i = s.allIndexOf('the', true) // result [ 0, 34 ]
    show('s', 'the', i, true);

    i = s.allIndexOf('THE', true) // result [ 0, 34 ]
    show('s', 'THE', i, true);

    r.append('<br><span class="var">var</span> t = <span class="str">"' + t + '"</span>;<br>');

    i = t.allIndexOf(1); // result [ 14, 58 ]
    show('t', 1, i);

    r.prepend('<br>');
    p = true; // prepend user added examples
    $('button').click(function() {
        var st = $('.string').val(),
            ss = $('.search').val(),
            ic = $('.case').prop('checked');
        show('"' + st + '"', ss, st.allIndexOf(ss, ic), ic);
    });

});

<h1 class="fn">String.allIndexOf()</h1>
<h2><em>String.allIndexOf(searchstring, ignoreCase)</em></h2>

<ul>
    <li>String <span class="str">[String]</span> - the string to search within for the searchstring</li>
    <li>searchstring <span class="str">[String]</span> - the desired string with which to find starting indexes</li>
    <li>ignoreCase <span class="bool">[Boolean]</span> - set to true to make both the string and searchstring case insensitive</li>
</ul>

<h2>Test it Out</h2>
<div id="form">
    <label>String:</label> <input class="string" type="text" value="Zombie ipsum reversus ab viral inferno, nam rick grimes malum cerebro."><br>
    <label>Search for:</label> <input class="search" type="text" value="er"><br>
    <button>Test</button>
    <label>Ignore case:</label> <input class="case" type="checkbox">
</div>

<h2>Examples of use</h2>
<pre id="results"></pre>

body { background: #333; color: #ddd; margin: 0 8px; }
/* Demo only css */
#form, pre { margin-left: 30px; font-size: 16px; }
#form { line-height: 25px; width: 300px; }
h1 { margin: 21px 0; font-size: 32px; font-weight: 700; font-family: 'Jura', sans-serif; }
h2 { margin: 19px 0; font-size: 24px; font-weight: 700; }
em { font-style: italic; }
ul { padding-left: 40px; margin: 16px 0; }
li { list-style-type: disc; }
input.string, input.search { width: 200px; float: right; }
.var { color: #ddd; }
.fn { color: #e9b965; }
.cmt { color: #999; }
.str { color: #92d050; }
.bool { color: #f00; }
.num { color: #b22; }