CODEWITHSUNDEEP
javascriptarrays
kimli
kimli

Reputation: 269

How to sum objects' property within an array in Javascript

I have an array of objects, and I'm trying to sum all the nums in the array. The array looks like this: 

var arr = [{"year": 2011, "num": 0}, {"year": 2012, "num": 2}, {"year": 2013, "num": 5}];

So the sum should be 7.

How do you calculate the total in Javascript?

I tried:

var sum = arr.reduce(function(a, b) {
    return a + b;
});

But this returns:

"[object Object][object Object][object Object]"

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

Upvotes: 0

Views: 242

Answers (6)

elixenide
elixenide

Reputation: 44851

You just need to loop over the array and increment a variable:

var arr = [{"year": 2011, "num": 0}, {"year": 2012, "num": 2}, {"year": 2013, "num": 5}];
var sum = 0;
for(var obj in arr) {
    sum += arr[obj].num;
}
// sum now contains the value 7

Here's a working jsFiddle to demonstrate.

Upvotes: 0

Tom Fenech
Tom Fenech

Reputation: 74695

Sounds like you want to reduce an array of values to a single value. Use reduce:

var a = [{"year": 2011, "num": 0}, {"year": 2012, "num": 2}, {"year": 2013, "num": 5}];
a.reduce(function(prev, curr) { return prev + curr.num }, 0);
7

reduce goes through each element of the array and accumulates a value using the function you pass it. The last argument is the initial value of prev.

This has "only" been in the ECMAScript Language specification since 2011, so if you need to support old browsers, you might like to try something like underscore.js:

_.reduce(a, function(prev, curr) { return prev + curr.num }, 0);

This will fall back to the native implementation if such a thing exists.

Or if you like to roll your own, you can use a loop:

for (var i = 0, sum = 0; i < a.length; ++i) {
    sum += a[i].num;
}

Upvotes: 4

Andrew Vermie
Andrew Vermie

Reputation: 633

You can use Array.prototype.map to turn it into an array of numbers, then Array.prototype.reduce to sum them:

var array = [{"year": 2011, "num": 0}, {"year": 2012, "num": 2}, {"year": 2013, "num": 5}];
var sum = array.map(function (a) {
    return a.num;
}).reduce(function (a, b) {
    return a + b;
});

Upvotes: 1

Rodrigo Pereira
Rodrigo Pereira

Reputation: 1924

var arr = [{"year": 2011, "num": 0}, {"year": 2012, "num": 2}, {"year": 2013, "num": 5}];
var sum = 0;

arr.forEach(function(obj){
    sum += obj.num;
});

//now sum have 7
console.log(sum);

Upvotes: 1

martskins
martskins

Reputation: 2940

This should do it...

function getSum(theArray){
    var sum = 0;
    for(var i = 0; i<theArray.length; i++){
        var sum += theArray[i].num
    }

    return sum;
}

Upvotes: 1

Paul Facklam
Paul Facklam

Reputation: 1643

var arr = [{"year": 2011, "num": 0}, {"year": 2012, "num": 2}, {"year": 2013, "num": 5}];
var sum = 0;

for(var i = 0; i < arr.length; i++) {
    sum = sum + arr[i].num;
}

Upvotes: 3

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