Finding patterns in a string
Here are two different ways of doing this, one using repeated calls to .indexOf() and one using repeated calls with a regex to .exec().  The regex solution requires the pattern argument to be a regex compatible string or an actual RegExp object with the g flag on it.

The .indexOf() option should be much faster than what you have since it avoids repeated .slice() operations and .indexOf() as a library function should be faster than your own variation.  The regex option is a lot more flexible since you can search for phrases that match a regular expression.



function find1(pattern,str) {
    var  arr = [];
    var i = 0;
    while ((i = str.indexOf(pattern, i)) &gt; -1) {
        // got a match starting at i
        arr.push([i, i + pattern.length - 1]);
        i++;
    }
    return arr.length ? arr : false;
};

function find2(pattern,str) {
    var  arr = [], matches;
    if (typeof pattern === "string") {
     pattern = new RegExp(pattern, "g");
    } else if (!(pattern instanceof RegExp)) {
        throw new Error("pattern must be string or RegExp object");
    }
        
    while (matches = pattern.exec(str)) {
        arr.push([matches.index, matches.index + matches[0].length - 1]);
    }
    return arr.length ? arr : false;
};


document.getElementById("run1").addEventListener("click", function(e) {
    var results = find1('abfd','abfdffdabfdfaffab');
    log(results);
});

document.getElementById("run2").addEventListener("click", function(e) {
    var results = find2('abfd','abfdffdabfdfaffab');
    log(results);
});

function log(x) {
    var d = document.createElement("div");
    d.innerHTML = JSON.stringify(x) + "&lt;br&gt;";
    document.body.appendChild(d);
}
&lt;button id="run1"&gt;Run .indexOf()&lt;/button&gt; 
&lt;button id="run2"&gt;Run regex&lt;/button&gt;&lt;br&gt;&lt;br&gt;