Is the empty string a subtring of all strings? [duplicate]

Question

I've been designing a search bar function that removes results that do not include the value of the user input. So far it's been successful, but when the user enters an empty string, I want it to return back to default and reverse all of the hidden posts. And while it does work, I wanted to ask the question if all strings contain a substring "" which is empty as it would help my understanding.

Here is what I mean:

let strArray = ['Hello', 'w', '324551', ''];

for (var i=0; i<strArray.length; i++) {
    console.log(strArray[i].indexOf("") !== -1);
}

Output: 
    True
    True
    True
    True

This may be a simple question to answer for some, but I'd just like clarification to reason my work.

Answer 1

Yes it is. From the spec:

2. If searchValue is the empty String and fromIndex ? len, return fromIndex.

Where fromIndex is almost always 0. (It will only not be 0 if you pass a second optional parameter to indexOf)