Mastering JavaScript Functional Programming - Second Edition

A reduce() example

Let's look at a practical example. Suppose a service has returned a JSON object, which itself has an array of objects containing an account id and the account balance. How can we get the list of IDs that are in the red, with a negative balance? The input data could be as follows:

const serviceResult = {  accountsData: [    {      id: "F220960K",      balance: 1024,    },    {      id: "S120456T",      balance: 2260,    },    {      id: "J140793A",      balance: -38,    },    {      id: "M120396V",      balance: -114,    },    {      id: "A120289L",      balance: 55000,    },  ],};

We could get the delinquent accounts with something like the following. You can check that the value of the delinquent variable correctly includes the two IDs of accounts with a negative balance:

const delinquent = serviceResult.accountsData.filter( ...