strstr.js

function _strstr(str1, str2){
    var i = 0;
    for(var c of str2) {
        if(str1[i++] != c) {
            return false;
        }
    }
    return true;
}

function* strstr(str1, str2) {
    var last = str1;
    var i = 0;
    while(last.length >= str2.length) {
        if(_strstr(last, str2)) {
            yield i;
        }
        i++;
        last = last.substr(1);
    }
    return -1;
}

var s = "122wasdasds121";

var gen = strstr("a1f123abc123ce123aaa123456123as1112233123333", "123");

for(var i of gen) {
    console.log(i);
}