有人可以用简单的英语解释我的对象代码中特定部分的情况吗？

Question

although it is a very simple code, I would like to get a full understanding of what is happening in my condition:

let getFreqOn = function(string){

    //set a variable for object

    let object = {}

    for (let key = 0; key < string.length; key++){

        // if (object.hasOwnProperty(string[key])) {

        // if (object[string[key]]) {

        // if (object[string[key]] !== undefined) {  

        if (string[key] in object) { 

            object[string[key]]++
        }
        else{
            object[string[key]] = 1
        }
    }
    return object
}

My main concern would be the first condition, I understand what it is they do but I cant put in to plain English how it is working. For example if (string[key] in object) is basically telling my that if a specific property is in the empty object I defined, then I will set then it will be set as the property and incremented. But what I'm trying to wrap my head around is that the object is empty, so how can the property be in the object?

Hoping someone can enlighten me on the conditions that I commented out as well. Sorry for the noob question.

Answer 1

First, the in operator returns a boolean result. It checks whether the string on the left is present as a property name in the object on the right.

Thus

if (string[key] in object)

asks whether that single character of the string is in use as a property name in the object. As you observed, the very first time through the loop that cannot possibly be true, because the object starts off empty.

Thus the if test is false , so the else part runs. There, the code still refers to object[string[key]] , but it's a simple assignment. An assignment to an object property works whether or not the property name is already there; when it isn't, a new object property is implicitly created.

The key difference is right there in the two different statements from the two parts of the if - else :

  object[string[key]]++; // only works when property exists

  object[string[key]] = 1; // works always