js对象和if语句。 为什么此代码在没有else且不使用else或else if语句的情况下有效？

Question

I am a very beginner JS "developer" (student) and I have run into a question I haven't been able to solve: why does the value of the repeated letters in my 'helperHash' increase when I am not using an else statement and why this same value DOESN't increase if I use an else statement? My code runs as expected but I am having problems understanding the logic behind this issue...

The code is supposed to return an array with the letters that have 1 or more repetitions in the given str.

function nonUniqueLetters(str){
  var strToChars = str.split('');
  var finalArr = [];
  var helperHash = {};
  for (let i = 0; i < strToChars.length; i += 1){
    let char = str[i];
    if (!helperHash[char]){
      helperHash[char] = 0;
    }
    helperHash[char] += 1;  //HERE! why doesn't this work if inside an else?
  }
  for (var key in helperHash){
    if (helperHash[key] > 1){
      finalArr.push(key);
    }
  }
  return finalArr;
}

Answer 1

For helperHash[char] …

The initial value is undefined and !undefined is true so it sets the value to 0 .

The next time char has the same value, helperHash[char] is 0 and !0 is also true so it sets the value to 0 (which it already is, so it makes no difference).

---

Instead of testing if the value is a false value, test if it is undefined, or if it exists at all.

if (typeof helperHash[char] === "undefined")

or

if (char in helperHash) 

Answer 2

The reason is this if (!helperHash[char]){ and how integers are converted to booleans in Javascript.

You initialize every member of the hash to 0 which equals a boolean false, so the else is never hit because helperHash[char] === 0 === false and thus !helperHash[char] is true for all values initialized with 0

Answer 3

Logic error.

How the current code works:

if (!helperHash[char]){
    // Enters here only when helperHash[char] is not set (or 0, but it is never 0)
    helperHash[char] = 0;
}
// Always increment
helperHash[char] += 1;
// There is no 0 in helperHash at this point

Why putting helperHash[char] += 1 in an else branch doesn't work:

if (!helperHash[char]){
    // Enters here only when helperHash[char] is not set or 0
    helperHash[char] = 0;
    // Still 0, will take this branch again on the next occurrence of char
} else {
   // Increment only if it was already 1 or more (this never happens)
   helperHash[char] += 1;
}
// Here helperHash contains only 0 for all keys

How to make it work:

if (!helperHash[char]){
    // This is the first occurrence of char, let's count it
    helperHash[char] = 1;
} else {
    // This is a subsequent occurrence of char, let's count it too
    helperHash[char] += 1;
}
// There is no 0 in helperHash at this point

Answer 4

Your if condition:

!helperHash[char]

is always evaluating to true ( helperHash never has characters in it that are "falsy"). So, an else branch of that if would never be hit.