JS How to make a reduce to sum a index of a array?

Question

I need help to learn how to make a reduce to a array like this:

a=[ ["ww","fsf",40],["ww","fsf",30],["ww","fsf",10]]

I want to learn how to sum the 40+30+10 using the method reduce. I know how to do it if the array was [40,30,10]. Can anyone help me? Thank you.

Andreas Louv · Accepted Answer · 2016-05-28 02:37:25Z

4

.reduce applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value:

a.reduce(callback[, initialValue])

Your values will be arrays:

["ww","fsf",40]
["ww","fsf",30]
["ww","fsf",10]

So you can use:

a.reduce(function(sum, value) {
  return sum + value[2];
}, 0);

Or using ES6 new arrow functions:

 a.reduce((sum, value) => sum + value[2], 0);

answered May 28, 2016 at 1:11

Andreas Louv

47.3k14 gold badges109 silver badges126 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Thank you. That was very usefull.

You're missing the initial value in the arrow function example.

@torazaburo Thanks for notifying me :-)

pfooti · Accepted Answer · 2016-05-28 00:59:57Z

2

a.reduce((sum, row) => sum + row[row.length - 1], 0)

answered May 28, 2016 at 0:59

pfooti

2,7344 gold badges27 silver badges40 bronze badges

Can you explain the => ? Can you write the same using the syntax function(){} inside the reduce? Thank you.