JSFiddle - React, Tailwind, and code Playground

HTML

<div ng-app='converterApp' ng-controller='converterCtrl'>Meters
    <input type="text" ng-model="m" />
    <br>Kilometers
    <input type="text" ng-model="km" />
</div>

JavaScript

'use strict';

angular.module('converterApp', [])
  .controller('converterCtrl', function ($scope) {
    $scope.m = 7000;

    $scope.$watch('m', function (v) {
        $scope.km = v / 1000;
    })

    $scope.$watch('km', function (v) {
        $scope.m = v * 1000;
    });
});