How to sum values from an array of key-value pairs in JavaScript?

To sum values from an array of key-value pairs in JavaScript, we use the map and reduce methods.

For instance, we write

const myData = [
  ["2013-01-22", 0],
  ["2013-01-29", 0],
  ["2013-02-05", 0],
  ["2013-02-12", 0],
  ["2013-02-19", 0],
  ["2013-02-26", 0],
  ["2013-03-05", 0],
  ["2013-03-12", 0],
  ["2013-03-19", 0],
  ["2013-03-26", 0],
  ["2013-04-02", 21],
  ["2013-04-09", 2],
];
const sum = myData.map(([, v]) => v).reduce((sum, current) => sum + current, 0);

to call myData.map with a callback that gets the 2nd value from each nested array.

Then we call reduce to return the sum of sum and current and set the initial sum to 0.