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This is an helper library for writing validators.

What is a validators

A validator is a function. When it runs against an object, it returns true or false.

var isFive = function(o) {
  return o === 5;
isFive(5); // true
isFive(4); // false

Writing complex validators may be a bit verbose. This library helps you to write short yet expressive validators. It also takes care of assigning the function a sensible name, this can be very helpful for debugging. For example:

match([1,'test']).name === 'array:[isNumber:1,isString:test]'

Importing the library

This library contains a main helper called "match" and some extra ones.

var match = require('occamsrazor-match');
var has = require('occamsrazor-match/extra/has');
var isInstanceOf = require('occamsrazor-match/extra/isInstanceOf');
var isPrototypeOf = require('occamsrazor-match/extra/isPrototypeOf');
var not = require('occamsrazor-match/extra/not');
var or = require('occamsrazor-match/extra/or');
var and = require('occamsrazor-match/extra/and');
var some = require('occamsrazor-match/extra/some');
var every = require('occamsrazor-match/extra/every');
var someValues = require('occamsrazor-match/extra/someValues');
var everyValues = require('occamsrazor-match/extra/everyValues');
var greaterThan = require('occamsrazor-match/extra/greaterThan');
var lessThan = require('occamsrazor-match/extra/lessThan');
var isUndefined = require('occamsrazor-match/extra/isDefined');
var isDefined = require('occamsrazor-match/extra/isUndefined');
var isNumber = require('occamsrazor-match/extra/isNumber');
var isString = require('occamsrazor-match/extra/isString');


This is the main helper, and can be used to create a lot of different validators.

These ones matches the value in it.

var isFive = match(5);
var isNull = match(null);
var isTrue = match(true);
var isFalse = match(true);
var isHello = match('hello');

For example:

isHello('hello'); // true
isFive(5); // true

Using undefined you create a validator that matches any value.

var isAnything = match(undefined);
isAnything({ greeting: 'hello' });

To match undefined there is an helper (isUndefined), explained below. Also if you wish to use a more explicit style, you can use the isAnything helper.

Using a regular expression, the validator will run that, on the value.

var doesMatch = match(/[0-9]+/);
doesMatch('123'); // true

Passing a function, the function itself will be returned:

var isLessThan5 = match(function (n) {
  return n < 5;
isLessThan5(2); // true
isLessThan5(8); // false

It looks like there is no real reason to do it, this will make more sense in a bit.

match - recursion

"match" can take an array or an object and perform a nested validation:

var isPoint = match({ x : isNumber, y: isNumber });
isPoint({ x: 1, y: 2 }); // true
isPoint({ x: 1, y: 2, z: 3 }); // true
isPoint({ x: 1, z: 3 }); // false

Passing values as undefined indicates that I don't really care about their value. Or if they are defined

var isPointOnXaxis = match({ x : 0, y: undefined });
isPointOnXaxis({ x: 1, y: 2 }); // false
isPointOnXaxis({ x: 0, y: 2 }); // true
isPointOnXaxis({ x: 0 }); // true

Values will be interpreted as specified before so I can implement recursive validation:

function isSalary(n) { return typeof n === 'number' && n > 0; }
var isEmployee = match({
  name : /[A-Za-z]+/,
  job: {
    position: undefined,
    salary: isSalary

Arrays are supported as well:

var startsWith123 = match([1, 2, 3]);
startsWith123([1, 2, 3, 4]); // true
startsWith123([2, 3, 4]); // false

In Arrays the behaviour of undefined comes useful:

var thirdArgIsNumber = match([undefined, undefined, isNumber]);
thirdArgIsNumber(['a', 'b', 3, 4]); // true
thirdArgIsNumber(['a', 'b', 'c']); // false


This checks for the existence of a list of attributes:

var isPoint = has(['x', 'y']);


It checks if an object has been built with a specific factory function:

var isPoint = isInstanceOf(Point);


It checks if an object is the prototype of another:

var isPoint = isPrototypeOf(Point.prototype);


Negate the result of a validator. I can take either a function or the argument used for the match function.

var isNotAPoint = not(isPrototypeOf(Point.prototype));
var isNotFive = not(5);


A validator that returns true only if all validators passed as argument return true. Every validator passed is transformed into a function using "match".

function isOdd(n) { return n % 2 !== 0; }
function isInteger(n) { return n % 1 === 0; }
function isSquares(n) { return isInteger(Math.sqrt(n)); }
var oddAndSquared = and([isOdd, isSquared]);
oddAndSquared(4); // false
oddAndSquared(9); // true


A validator that returns true if at least one validator passed as argument returns true. Every validator passed is transformed into a function using "match".

var is5or9 = or(5, 9);
is5or9(5); // true
is5or9(9); // true
is5or9(3); // false

some/every and someValues/everyValues

some/every validates arrays. It takes a validator as argument (it uses "match" behind the scene) and checks the array items against that validator. In the case of "some", at least a check should pass to return true. In the case of "every", all of them should pass.

var atLeastOne5 = some(5);
atLeastOne5([1, 2, 5]); // true
atLeastOne5([5, 5, 2]); // true
atLeastOne5([1, 2, 3]); // false
var allNegatives = every(function isNegative(n) { return n < 0; });
allNegatives([-1, -2, -5]); // true
allNegatives([-5, 5, -2]); // false

someValues and everyValues do the same on all values of an object.

greaterThan, lessThan

They are be useful to validate if a number is in a specific range.


var isGreaterThan0 = greaterThan(0);
isGreaterThan0(1); // true
isGreaterThan0(-1); // false
var isValidDiscount = and(greaterThan(0), lessThan(100));

Common validators

The library includes a series of very common validators:

  • isUndefined
  • isDefined
  • isString
  • isNumber
  • isAnything (always returns true)


var isEmployee = match({
  id: isDefined,
  name: isString,
  salary: isNumber,
  title: or(isUndefined, 'mr', 'ms')

Custom validator

A validator is a simple function that given a value returns true or false. You can build your own and use it together with the other functions:

function isEven(n) {return o % 2 === 0;};
or([isEven, match(null)]);
  numberOfShoes: isEven

Note: I have used a named function (not an anonymous function expression or an arrow function). It is important as logging and debugging rely entirely on the function name.

Mixing them up!

Being able to use functions, you can mix-up and reuse validators:

var isPoint = match({ x : undefined, y: undefined });
var isSquare = match([isPoint, isPoint, isPoint, isPoint]);
var isTriangle = match([isPoint, isPoint, isPoint]);
var containsSquareAndTriangle = {
  triangle: isTriangle,
  square: isSquare,
  extra: or([isSquare, isTriangle])

validator names

Helpers provide always a descriptive name to the generated functions:

match(true).name === 'isTrue'
match(2).name === 'isNumber:2'
match([1,'test']).name === 'array:[isNumber:1,isString:test]'
has('test1', 'test2').name === 'object:{test1:isAnything,test2:isAnything}'

This can be very helpful for debugging.

Validation errors

This feature can be useful to have some insight about what went wrong in the validation. When validating a value you can pass a function, as a second argument. This function is called whenever a validation step fail (only if it is relevant to the validation):

var validator = match({
  user: {
    name: /[a-zA-Z]+/,
    jobtitle: or(['engineer', 'analyst'])
  deleted: not(true)
  user: {
    name: 'Maurizio',
    jobtitle: 'engineer'
  deleted: true
}, function (val) {
  // val is an object containing:
    path: path, // if this validation step is nested in an object or array
    name: name, // name of the function
    value: o // the value on which the validator ran


{ path: 'deleted', name: 'not(isTrue)', result: true, value: true }]);

If you want to collect all this informations in an array you can use the validationErrors helper:

var validationErrors = require('occamsrazor-match/extra/validationErrors');
var errors = validationErrors();
var validator = arrayOf(5);
validator([5, 5], errors);
console.log(errors()); // returns a list of all errors

Validate decorator

A very common use case for validation is checking if the arguments of a function match a certain criteria. Because of this I have included a function decorator that checks the arguments of the decorated function and throws an error if they don't match:

var validate = require('occamsrazor-match/validate-decorators');
var sum = validate(isNumber, isNumber, isNumber)(function sum(a, b, c) {
  return a + b + c;

if supported you can use the succint es7 syntax:

@validate(isNumber, isNumber, isNumber)
function sum(a, b, c) {
  return a + b + c;

The error object contains a description of all the things went wrong:

try {
  var result = sum('1', 2, 3);
} catch (e) {
  console.log(e.message); // Function called with wrong arguments: array:[isNumber,isNumber,isNumber]
  console.log(e.errors); // [{ path: [0], name: 'isNumber', value: '1' }]

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