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2.0.3 • Public • Published


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Set of various predicates for type checking, simple assertions, filtering etc.


  • written in typescript (with type guards and function overloading)
  • well defined API design principles
  • predicates description every predicate contains a proper description for easier debugging and automatic assertion generation
  • supports simple type checks with unnecessary verbosity
  • every predicate supports currying if possible


npm install predicates

Full API


const is = require('predicates');
is.string(1); // false
is.string('test'); // true
is.undefinedOr(String, undefined); // true
is.undefinedOr(String, 'string'); // true
is.undefinedOr(is.string, undefined); // true
is.undefinedOr(is.string, 'timmy'); // true
is.undefinedOr(is.string)(undefined); // true
is.undefinedOr(is.string)('timmy'); // true
const isPerson = is.structure({
    name: is.string,
    surname: is.undefinedOr(is.string),
    age: is.number
isPerson({name: 'Tom', age: 10}); // true
isPerson({surname: 'Welling', age: 100}); // false, lack of name property
const assertName = is.assert(is.string);
const assertSurname = is.assert(is.notBlank);
const assertAge = is.assert(is.undefinedOr(is.positive));
const Person = function(name, surname, age) {
new Person('Tom', 'Welling', 33); // OK!
new Person('Tom', 'Welling'); // OK!
new Person('Tom', '', 33); // Error: Surname must be a string and cannot be emptye

API design

Generated predicates can be called with more than one argument

Most of type checking, utility libraries force you use predicates with only one argument but predicates doesn.t Additionally predicates preserves the context of function call which allows you to create even more powerful logic.

const is = require('predicates');
const isOkToModifyTags = is.all(
    function(tags, previousTags) {
        // no need to save them again if they are the same as previous ones
        return tags.join(',') !== previousTags.join(',');
Module.prototype.modifyTags = function(entityId, tags) {
    var previousTags = getTags(entityId);
    if (isOkToModifyTags(tags, previousTags)) {
        this.saveTags(entityId, tags);
    } else {
        // no need to save them again if they are the same as previous ones

Prevents stupid mistakes (fail fast)

Predicates checks whether generated predicate is misconfigured and throws an error as fast as possible.

is.startsWith(''); // Error: Prefix cannot be empty
// since that would be true for all strings[]); // Error: Collection cannot be empty
// always false

That's why it's a good practice to create predicates at the beginning of a module definition to quickly catch any mistake.

// some module
const is = require('predicates');
const isImage =[]); // Error: Collection cannot be empty
// You don't need to run the whole application to get the error that your predicate is wrong
export class Module {

Defined and generated predicates will never throw any error

You don't need to check the arguments provided to predicates to make sure they won't cause an error - predicates does it for you.

const isDuck = is.hasProperty('quack');
isDuck(undefined); // false - no error
isDuck(1); // false - no error
isDuck('duck'); // false - no error
is.matches(/.*\.ts/)(100); // false - no error

NOTE! This rule applies only for predicates defined in the library. Any user-defined predicate MAY throw errors (however I don't advise to do so) and predicates will not catch them.

const assertName = is.all(is.string, function(value) {
    if (value === 'admin') {
        throw new Error('Admin is a reserved user name');

assertName('admin'); // Error: Admin is a reserved user name

Type guards

Every predicate (if possible) is a type guard

if (is.string(value)) {
   // at this point typescript compiler knows that value is a string 

Core types are automatically translated to proper predicate'foo', is.string)({foo: 'bar'}); // true
// for less verbosity this is possible as well'foo', String)({foo: 'bar'}); // true
is.arrayOf(String)(['foo']); // true


npm i predicates

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