Javascript strstr

strstr.js

String.prototype.strstr = function(needle) {
    const haystack = this;
    if(!needle) return [0];
    if(!haystack || needle.length > haystack.length) return [-1];

    let i,j;
    let positions = [];
    for(i=0;i<haystack.length;i++) {
        let index = i;
        j=0;
        while(haystack[index] == needle[j]) {
            if(j == needle.length-1) {
                positions.push(i);
            }
            index++;j++;
        }
    }
    return positions.length>0?positions:[-1];
}