How do I assign my .reduce function output to a variable?

Question

Given an object and a key, getElementsThatEqual10AtProperty returns an array containing all the elements of the array located at the given key that are equal to ten.

I want to assign what my .reduce method outputs to a variable then use that variable inside my if statement. My code evaluates to NaN .

function getElementsThatEqual10AtProperty(obj, key) {
  var arrayTotal = 0;
  obj[key].reduce(function(a,b){
    arrayTotal = a + b;
   });
  if(arrayTotal === 10) {
    return obj[key];
  }
};

Answer 1

You're approaching reduce incorrectly. You're almost using it like Array#forEach . Remember that callbacks are functions. They expect to return an actual value.

When you read the docs, you'll see that reduce also needs an accumulator , that is an argument after the callback.

const sum = obj[key].reduce(function(a,b){
  return a + b
}, 0)

Or if you want the newer and prettier version:

const sum = obj[key].reduce((a,b) => a + b, 0)

When you have a one liner, return is implicit, which means it's implied, as opposed to explicit.

The accumulator is what your value starts at while it totals up the values.

Array.reduce(callback, accumulator)