json_path.js

function compile(selector) {
  
  function identity(x) {
    return x
  }

  function compose(f, access) {
    return function(x) {
      let r = f(x)
      if (r && access in r) return r[access]
      else return null
    }
  }

  let f = identity
  
  let readingFieldAccess = false
  let readingArrayAccess = false
  let fieldName = ''
  let arrayIndex = ''
  let escaped = false
  for (let c of (selector + '.')) {
    if (readingFieldAccess && readingArrayAccess) {
    	throw new Error('Illegal state!');
    } else if (readingFieldAccess) {
    	if (c == '\\' && !escaped) {
      	escaped = true
      } else if ((c == '[' || c == '.') && !escaped) {
      	if (fieldName == '') throw new Error('Empty field name')
        f = compose(f, fieldName)
        fieldName = ''
        readingFieldAccess = c != '['
        readingArrayAccess = c == '['
      } else {
      	escaped = false
      	fieldName += c
      }
    } else if (readingArrayAccess) {
    	if (c == ']') {
      	if (arrayIndex == '') throw new Error('Empty array index')
        f = compose(f, parseInt(arrayIndex))
        arrayIndex = ''
        readingFieldAccess = false
        readingArrayAccess = false
      } else if (/\d/.test(c)) {
      	arrayIndex += c
      } else {
      	throw new Error('Illegal array index!')
      }
    } else if (!readingFieldAccess && !readingArrayAccess) {
    	if (c == '.') readingFieldAccess = true
      else if (c == '[') readingArrayAccess = true
      else throw new Error('Invalid expression!')
    }
  }
  
  return f
}

let obj = {
	passengers: [
  	{
    	flights: [
      	{
        	segments: [
          	{
            	services: {
              	seat: {
                	number: '1A'
                }
              }
            },
          	{
            	services: {
              	seat: {
                	number: '2A'
                }
              }
            }
          ]
        }
      ]
    }
  ]
}

function t(selector, obj) {
	let compiled = compile(selector)
	console.log(selector, obj, compiled, compiled(obj))
}

t('.passengers[0].flights[0].segments[1].services.seat.number', obj)
t('.passengerss.bla', obj)
t('.', obj)