Javascript字典键存在于条件语句中

Question

I'm encountering some strange behavior while working with a dictionary object in Node.JS as part of a MEAN stack project.

I define a keywordSearches dictionary earlier in my code, and searches is an array of Search objects that contains a keyword property. I'm essentially pulling from MongoDB a record of all my search requests, and then creating a dictionary containing the frequency of the keyword searches, with the key being the search text, and the value being the frequency of it being searched (an integer). All this is stored in keywordSearches .

However, when I use the following code to iterate through my searches, I see the keywords in keywordSearches evaluates to false outside of my if condition, but then apparently evaluates to true within the if condition (the very next line!). Why is this occurring?

  console.log(keywordSearches);
   for (var i = 0; i < searches.length; i++){
     var keywords =  searches[i].searchBody.keywords;
     console.log(keywords in keywordSearches); // <- this evaluates to false
     if (!keywords in keywordSearches){ // <- this section of code never executes! Why?
       console.log("New keyword found")
       keywordSearches[keywords] = 1; 
     } else {
       keywordSearches[keywords] = keywordSearches[keywords] + 1;
       console.log("else statement")
     }
   }
   console.log(keywordSearches);

Output (note, I have four Search objects, all with keyword "photography":

{}  <- console.log(keywordSearches)
false <- (keywords in keyWord Searches)
else statement <- if condition evaluates to false! Should evaluate to true. Why?
true
else statement
true
else statement
true
else statement
true
else statement
{ photography: NaN }

I understand why photography is NaN : it's never initialized with a value of 1 . (which it should if it's initially not found in the dictionary). So it's adding NaN + 1 each time.

Answer 1

in has lower precedence than ! , so your expression is evaluated as:

(!keywords) in keywordSearches

Instead of:

!(keywords in keywordSearches)

See: Operator precedence on MDN

Answer 2

Avoid using ! completely, and switch the if-else statements instead:

 console.log(keywordSearches);
 for (var i = 0; i < searches.length; i++){
 var keywords =  searches[i].searchBody.keywords;
 console.log(keywords in keywordSearches); // <- this evaluates to false
 if (keywords in keywordSearches){ 
   keywordSearches[keywords] = keywordSearches[keywords] + 1;
   console.log("keyword already exists")
 } else {
   console.log("New keyword found")
   keywordSearches[keywords] = 1; 
 }
}
console.log(keywordSearches);

This saves on an operation.