avj-test.js

var Ajv = require('ajv');
var ajv = new Ajv({allErrors: true});

var schema = {
    properties: {  
        country: { type: 'string', enum: ['RU', 'US', 'UZ'] },
        postalCode: { type: 'string', maxLength: 10 }
    },
    
    if: { 
        properties: {
            country: { enum: ['US'] }
        } 
    },
    then: {
        properties: {
            postalCode: { type: 'string', pattern: '^\\d{5}(-\\d{4})?$' },
        }
    },
    
    required:[ 'country', 'postalCode' ] 
};

var validate = ajv.compile(schema);

test({country: 'UZ', postalCode: '00000'});
test({country: 'US', postalCode: 'ZZZZZ'});
test({country: 'US', postalCode: '00000'});

function test(data) {
  var valid = validate(data);
  if (valid) console.log('Valid!');
  else console.log('Invalid: ' + ajv.errorsText(validate.errors));
}