Add up property from array of objects

Question

I am trying to add up the ID property from an array of objects. The result displayed is 01395 - instead, it should be 27 (as that's what 13 + 9 + 5 equals).

I believe the code is concatenating the ID properties rather the desired outcome of adding them.

var collection = [{
  "Name": "Charlie",
  "ID": "13"
}, {
  "Name": "Emma",
  "ID": "9"
}, {
  "Name": "Bob",
  "ID": "5"
}];

total = 0, //set a variable that holds our total
  id = collection, //reference the element in the "JSON" aka object literal we want
  i = 0;
for (i = 0; i < id.length; i++) { //loop through the array
  total += id[i].ID; //Do the math!
}
console.log(total); //display the result

JsFiddle: https://jsfiddle.net/3r8vhfb2/

What is a "JSON array"? ID is not an "element", it's a "property". "putting each element next to each other" is called concatenation. That's what + does with strings. — user663031
– user663031, Commented Jun 25, 2016 at 19:12
I'm not the downvoter, but the behavior of + with strings vs. numbers is well-documented and covered in many questions here on SO, potentially making this question "not useful". For example, see the MDN page. — user663031
– user663031, Commented Jun 25, 2016 at 19:15

Rayon · Accepted Answer · 2016-06-25 19:33:38Z

1

Convert it to Number

Unary plus (+), The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already.

var collection = [{
  "Name": "Charlie",
  "ID": "13"
}, {
  "Name": "Emma",
  "ID": "9"
}, {
  "Name": "Bob",
  "ID": "5"
}];

var total = 0,
  id = collection;
for (var i = 0; i < id.length; i++) {
  total += +id[i].ID;
  //OR total += Number(id[i].ID);
}
console.log(total);

Or using Array#reduce :

var collection = [{
  "Name": "Charlie",
  "ID": "13"
}, {
  "Name": "Emma",
  "ID": "9"
}, {
  "Name": "Bob",
  "ID": "5"
}];

var total = collection.reduce(function(a, b) {
  return +(a.ID || a) + +(b.ID);
});
console.log(total);

edited Jun 25, 2016 at 19:33

answered Jun 25, 2016 at 19:08

Rayon

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3 Comments

The Codesee Over a year ago

I would like to thank you for posting this answer and showing me how to rectify the problem of concatenating the ID properties :)

Rayon Over a year ago

@TheCodesee, I'm glad it helped mate! Happy Coding

The Codesee Over a year ago

Which answer is better out of the two you posted, performance-wise?

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