Cannot reduce array inside of an object

Question

I am trying to reduce an array inside of an object. I am getting back

push is not a function

I have started my array as empty and created a add function to pass in as the first argument.

function add(a,b) {
        return a +b;
}

var navBarArray = [];
var listArray = [];

var mapping = {
  ".navbar": navBarArray,
  ".list-group": listArray
};

I tried this approach on the mapping object but it creates errors

var mapping = {
  ".navbar": Math.round(navBarArray.reduce(add,0) ),
  ".list-group": listArray
};

However, I get push is not a function back in my console.

Below is my function that passes values to the array. I can create a variable inside the function and reduce it there. However, that limits access to my variable and will bloat my function as I continue.

  Object.keys(mapping).forEach(function(selector) { 
      $(selector).hover(function(evt) {
        console.log('mapping',mapping);
        console.log('selector',selector);
        enteredTime = new Date();
      }, function() {
        var ctime = new Date();
        var time = (ctime.getTime() - enteredTime.getTime())/1000;
        mapping[selector].push(time);

        // *********** this works but not where I need it to*******
        var reduce = Math.round(navBarArray.reduce(add,0) );
        console.log(reduce);
        });
    })

Which line is getting the error? Calling reduce shouldn't report an error about push. — Barmar
– Barmar, Commented Sep 22, 2015 at 18:03
Maybe make sure that mapping[selector] is typeof "array"? — zero298
– zero298, Commented Sep 22, 2015 at 18:03
Result of reduce is not an array in your case. And it's not clear what you want it to be actually. — raina77ow
– raina77ow, Commented Sep 22, 2015 at 18:03
The second var mapping = ... snippet sets the value of the .navbar key to a number, but then you try to use it like an array with mapping[selector].push(...). I think you just need to stick with the first definition of mapping. — andersschuller
– andersschuller, Commented Sep 22, 2015 at 18:04

Barmar · Accepted Answer · 2015-09-22 18:09:47Z

2

Change your mapping object so it has separate places for the array and total:

var mapping = {
    ".navbar": {
        total: 0,
        times: []
    },
    ".list-group": {
        total: 0,
        times: []
    }
}

Then you do mapping[selector].times.push(time), and put the total with:

mapping[selector].total = mapping[selector].times.reduce(add, 0);

answered Sep 22, 2015 at 18:09

Barmar

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2 Comments

Winnemucca Over a year ago

Thank you! I follow this approach. From what I read I could not place a reduce in the object.

Barmar Over a year ago

You must have misread or misunderstood. There's nothing special about reduce, it's just a function call that returns a value. And you can put any value in an object.

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