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
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 propertyconst 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) {
assertName(name);
assertSurname(surname);
assertAge(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 emptyeMost of type checking, utility libraries force you to use predicates with only one argument but predicates doesn't. Additionally, predicates preserves the context of function calls which allows you to create even more powerful logic.
const is = require('predicates');
const isOkToModifyTags = is.all(
is.arrayOf(is.string),
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
}
}Predicates checks whether generated a 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
is.in([]); // Error: Collection cannot be empty
// always falseThat's why it's a good practice to create predicates at the beginning of a module definition to quickly catch any mistakes.
// some module
const is = require('predicates');
const isImage = is.in([]); // 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 {
}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 errorNOTE! 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
Every predicate (if possible) is a type guard
if (is.string(value)) {
// at this point typescript compiler knows that value is a string
}is.property('foo', is.string)({foo: 'bar'}); // true
// for less verbosity this is possible as well
is.property('foo', String)({foo: 'bar'}); // true
is.arrayOf(String)(['foo']); // true
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