JSFiddle - React, Tailwind, and code Playground

JavaScript

var data = [{
    "id": 5,
    "name": "a",
    "select": true
  },

  {
    "id": 3,
    "name": "b",
    "select": false
  },

  {
    "id": 2,
    "name": "x",
    "select": true
  },

  {
    "id": 1,
    "name": "y",
    "select": false
  }
]
console.log(data.filter((x,i) => { return x.select; }).length)