Add to JS object or increase count if key exists

Question

I'm trying to add a key to an object if it doesn't exist, or increase its count if it's already present. The following code correctly adds the new key if it doesn't exist, but doesn't increase its count if it already does. Instead it returns {UniqueResult1:NaN, UniqueResult2:NaN}.

let detectionHash = {};
    function onDetected(result) {
        detectionHash[result.code]++;
        if(detectionHash[result.code] >= 5) {
          //here's where I want to be
        }
    }

How can I increase the count of the key if it already exists?

complete code and how are you are using it to get the unexpected behaviour — ashish singh
– ashish singh, Commented Aug 11, 2018 at 19:36

Nina Scholz · Accepted Answer · 2018-08-11 19:39:53Z

11

You could take the value or the default value of zero and add one.

A not existing property returns an undefined, which is falsy. The following logical OR || checks this value and take the next value of zero for incrementing.

detectionHash[result.code] = (detectionHash[result.code] || 0) + 1;

edited Aug 11, 2018 at 19:39

answered Aug 11, 2018 at 19:35

Nina Scholz

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2 Comments

Michał Over a year ago

Would this not replace the object keys? What I'm really looking for is to store the count of each unique key. The second part (unique key) I figured out, just not the first part.

J. Pichardo Over a year ago

No, that would only change the value for that specific key.

Damián Pablo González · Accepted Answer · 2018-08-11 19:56:11Z

0

If you ask for a key that doesn't exists, it will be of "undefined" type:

var abc=[];
abc[5] = 'hi';
console.log(typeof abc[3]); //it will be "undefined", as a string (in quotes)

So:

let detectionHash = {};
    function onDetected(result) {
        //does the key exists?
        if (typeof detectionHash[result.code] !== "undefined") 
            detectionHash[result.code]++; //exist. increment
        else
            detectionHash[result.code]=0; //doesn't exist. create
        if(detectionHash[result.code] >= 5) {
          //here's where I want to be
        }
    }

edited Aug 11, 2018 at 19:56

answered Aug 11, 2018 at 19:45

Damián Pablo González

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1 Comment

Damián Pablo González Over a year ago

0 or 1. You don't say what value you want in your new key

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