编写一个函数isMerge，判断一个字符串str是否可以由其他两个字符串part1和part2“组合”而成

function isMerge(s,part1,part2){
    if(!(s.length == part1.length + part2.length)) {
        return false;
    }else{
        var index1=0;
        var index2=0;
        for(var i=0; i<s.length; i++){
            if(s[i] == part1[index1]){
                index1++;
            }else if(s[i] == part2[index2]){
                index2++;
            }else{
                break;
            }
        }
        return (index1==part1.length) && (index2==part2.length);
    }
}
console.log(isMerge("codewars","cdw","oears")); //true