filter-fields.js

foo = [ 
  { fields: [ 'a1', 'a2' ] },
  { fields: [ 'b1', 'b2' ] },
  { fields: [ 'c2' ] } 
]

// method 1: forEach

// fieldsThatEndWithOne = []
// foo.forEach(f => {
//   f.fields.forEach(field => {
//     if (field.endsWith('1')) fieldsThatEndWithOne.push(field)
//   })
// })

// method 2: map + lodash.flatten

// const { flatten } = require('lodash')
// fieldsThatEndWithOne = flatten(foo.map(f => {
//   return f.fields.filter(field => field.endsWith('1'))
// }))

// method 3: reduce

// fieldsThatEndWithOne = foo.reduce((acc, f) => {
//   return acc.concat(f.fields.filter(field => field.endsWith('1')))
// }, [])

// method 4: flat

const flat = require('flat')
fieldsThatEndWithOne = Object.values(flat(foo)).filter(value => value.endsWith('1'))

console.log(fieldsThatEndWithOne)