Deterrent

Deterrent is a type-safe, functional data validation library with no dependencies.

Assertable

The essential type exposed by Deterrent is an assertable.

An Assertable<T> is an object whose member assert is a function that accepts a parameter of type unknown and returns a value of type T or throws a ValidationError.

For example, an Assertable<string> looks like this:

{
	assert: (value: unknown) => string;
}

Any unvalidated value can be passed into the assert function. If the value passes all validation checks, it is returned as type string, since the assertable at least guarantees that value is a string. If the value is not a string, or fails any further validation checks (such as matching a regex pattern), the function will throw a ValidationError containing a message describing why the value was invalid.

API

NumberValidator

A validator for values of type number.

number(options)

Creates a NumberValidator.

options.name: string -- the name for the validator used in error messages. Defaults to "Value".

options.allowNumericString: boolean -- whether to allow numeric strings as input, which are immediately converted to type number. Defaults to true.

options.initialAssert -- see the Initial Assert section at the end.

NumberValidator.min(minValue: number, options)

Checks that the number is at least minValue.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

NumberValidator.max(maxValue: number, options)

Checks that the number is at most maxValue.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

NumberValidator.integer(options)

Checks that the number is an integer.

options.roundIfNot: boolean -- if the value is not an integer, rounds to the nearest integer and continues instead of throwing an error. Defaults to true.

options.divisibleBy: number -- requires that the value (after being rounded, if applicable) is divisible by the given number.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

NumberValidator.custom(validator)

See the Custom Validator Function section below.

NumberValidator.asString()

Converts the value to a string by calling toString(), and returns a StringValidator.

NumberValidator.assert(value)

Calls the validation chain; returns the value if it passes all checks, or throws a ValidationError if it fails.

StringValidator

A validator for values of type string.

string(options)

Creates a StringValidator.

options.name: string -- the name for the validator used in error messages. Defaults to "Value".

options.initialAssert -- see the Initial Assert section at the end.

StringValidator.minLength(minLength, options)

Checks that the string's length is at least minLength characters.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

StringValidator.maxLength(maxLength, options)

Checks that the string's length is at most maxLength characters.

options.truncate: boolean -- if true, removes excess characters. If false, and the string is longer than maxLength characters, an error is thrown. Defaults to false.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

StringValidator.pattern(pattern: RegExp, options)

Checks that the string satisfies the given pattern.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

StringValidator.custom(validator)

See the Custom Validator Function section below.

StringValidator.split(separator, options)

Splits the string by the given separator and returns an ArrayValidator<string>.

options.listName: string -- the name for the new array validator.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

StringValidator.assert(value)

Calls the validation chain; returns the value if it passes all checks, or throws a ValidationError if it fails.

BooleanValidator

A validator for values of type boolean.

boolean(options)

Creates a BooleanValidator.

options.name: string -- the name for the validator used in error messages. Defaults to "Value".

options.coerce: boolean -- whether to coerce the unknown value into a boolean using truthiness. If false, any non-boolean values will immediately cause an error to be thrown. Defaults to false.

options.initialAssert -- see the Initial Assert section at the end.

BooleanValidator.true(options)

Checks that the boolean is true.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

BooleanValidator.false(options)

Checks that the boolean is false.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

BooleanValidator.assert(value)

Calls the validation chain; returns the value if it passes all checks, or throws a ValidationError if it fails.

ArrayValidator<EL>

A validator for values of type EL[], i.e., arrays whose elements are of type EL.

array<INIT>(options)

Creates an ArrayValidator<INIT>. INIT will default to unknown[] unless options.initialAssert is specified. In either case, it should be inferred, so you'll rarely need to specify this type parameter.

options.name: string -- the name for the validator used in error messages. Defaults to "Value".

options.coerce: boolean -- whether to coerce the given value into an array. Defaults to false. If true, the initial assert function behaves as follows:

1. If the value is null or undefined, returns [] (an empty array).
2. If the value is anything else (except an array), returns [value] (i.e., an array with value as its only item).

options.initialAssert -- see the Initial Assert section at the end.

array.minLength(minLength, options)

Checks that the array has at least minLength elements.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

array.maxLength(maxLength, options)

Checks that the array has at most maxLength elements.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

options.truncate: boolean -- if true, removes excess elements and continues. If false, and the array contains more than maxLength elements, an error is thrown. Defaults to false.

of<NEXT>(assertable: Assertable<NEXT>, options)

Checks that the elements of the array pass the given assertable.

options.allOrNothing: boolean -- if true, an element that fails to pass the assertable will cause the entire check to fail. If false, the element will simply be omitted from the array. Either way, the resulting array will only contain elements that passed the given assertable. Defaults to true.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

custom(validator)

See the Custom Validator Function section below.

ArrayValidator.assert(value)

Calls the validation chain; returns the value if it passes all checks, or throws a ValidationError if it fails.

TupleValidator<TUP extends any[]>

A validator for tuple types, i.e., arrays with a fixed size that satisfy the type TUP.

options.name: string -- the name for the validator used in error messages. Defaults to "Value".

options.initialAssert -- see the Initial Assert section at the end.

tuple<INIT>(message?: string, options)

Creates a TupleValidator\<INIT>. INIT will default to any[] unless options.initialAssert is specified. In either case, it should be inferred, so you'll rarely need to specify this type parameter.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

TupleValidator.of<NEXT>(assertables, options)

Checks that the elements of the tuple exactly match the given assertables. If there are fewer elements than assertables, an error is thrown.

assertables is an array of assertables, where each element in value is checked against the assertable in its corresponding index.

options.throwIfExtra: boolean -- if true, throws if there are more elements than assertables. If false, the extra elements are simply omitted from the resulting array. Defaults to true.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

TupleValidator.assert(value)

Calls the validation chain; returns the value if it passes all checks, or throws a ValidationError if it fails.

ObjectValidator<SCHEMA extends Record<any, any>>

A validator for object types, i.e., anything that extends Record<any, any>. Even though null and arrays are technically considered objects, they will fail ObjectValidator's initial check.

object<INIT>(options)

Creates an ObjectValidator\<INIT>. INIT will default to Record<any, any> unless options.initialAssert is specified. In either case, it should be inferred, so you'll rarely need to specify this type parameter.

options.name: string -- the name for the validator used in error messages. Defaults to "Value".

options.initialAssert -- see the Initial Assert section at the end.

ObjectValidator.schema<NEXT>(assertableSchema)

Checks that the object specifies the given schema of assertables.

assertableSchema is a NEXT whose values are mapped to be assertables of their base types. For example, if you'd like to check that an object is of type

{ 
  name: string; 
  age: number;
}

you can call ObjectValidator.schema with the following argument:

{
  name: string(),
  age: number(),
}

For keys that are optional, use the optional utility validator (see below).

ObjectValidator.assert(value)

Calls the validation chain; returns the value if it passes all checks, or throws a ValidationError if it fails.

optional<TYPE>(assertable, options)

Wraps the given assertable, allowing null or undefined values. Do not specify type parameters as they will be inferred.

options.name: string -- name of optional assertable, used in error messages. Defaults to "Value".

options.defaultValue: TYPE -- a default value to use if the value is null or undefined. If no default value is provided, the respective value (null or undefined) will pass.

options.allowNull?: boolean -- whether to allow null values to pass. If false and no default value is provided, a value of null will cause the check to fail. Defaults to true.

options.allowUndefined?: boolean -- whether to allow undefined values to pass. If false and no default value is provided, a value of undefined will cause the check to fail. Defaults to true.

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

oneOf(assertables, options)

Wraps the given assertables, allowing values that satisfy any of the given assertables. Do not specify the type parameter as it will be inferred.

options.name: string -- the name of the oneOf assertable, used in error messages. Defaults to "Value".

options.errorMessage: string -- error message if this validation check fails. Defaults to a specific message intended to be read by an end user.

Initial Assert

All functions that create validators (number(), string(), etc) must provide an initial assert function. This function should accept a parameter of type unknown and return a type that extends its respective type. For example, an initial assert function for number() will typically return a number, but may return the type 1 | 2 | 3 | 4 | 5.

The default initial assert functions are appropriate for almost all use cases. In rare cases, you may want to combine validators of different types -- for example, taking the result from a NumberValidator, converting it to a boolean, and using that value for a BooleanValidator. In that case, pass to the BooleanValidator an initial assert function that calls NumberValidator.assert(), converts the result to a boolean, and returns it.

Custom Validator Function

To write your own check, simply pass in a validator function, which looks like this for a type T:

validator: (value: T, error: (message: string) => void) => T)

This function should perform any validation checks and return the value if it passes. If it fails, it should call error and pass in a descriptive error message, which will cause the entire validation chain to fail.