var test1 = "test12345";
var test2 = "1234";
var test3 = "";

var constraints1 = {
  length: {
    is: 4
  }
}

var constraints2 = {
  length: {
    minimum: 4
  }
}

var constraints3 = {
  length: {
    maximum: 4
  }
}

console.log('Testing for exact length');
console.log(validate.single(test1, constraints1));
console.log(validate.single(test2, constraints1));
console.log(validate.single(test3, constraints1));

console.log('Testing for min length');
console.log(validate.single(test1, constraints2));
console.log(validate.single(test2, constraints2));
console.log(validate.single(test3, constraints2));

console.log('Testing for max length');
console.log(validate.single(test1, constraints3));
console.log(validate.single(test2, constraints3));
console.log(validate.single(test3, constraints3));

[Log] Testing for exact length
[Log] ["is the wrong length (should be 4 characters)"]
[Log] undefined
[Log] undefined
[Log] Testing for min length
[Log] undefined
[Log] undefined
[Log] undefined
[Log] Testing for max length
[Log] ["is too long (maximum is 4 characters)"]
[Log] undefined
[Log] undefined