Newtonian Physics Mockumentary

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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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