type-assurer

type-assurer is a TypeScript-first type checking library, providing compatibility with lodash's type guard functions while ensuring type safety. Designed with ESModules in mind, it allows for tree-shaking to minimize bundle sizes.

Features

• Compatible with lodash type guard functions
• TypeScript-first implementation with accurate type inference
• ESModule ready for tree-shaking and bundle size optimization
• No external dependencies
• A collection of 7 type guard functions: a. isString - Similar to lodash's type guard functions b. assertString - Provides TypeScript's type assertion feature c. ensureString - Evaluates the argument's type and returns the value if the type guard passes, otherwise throws an exception d. fallbackString - Evaluates the first argument's type and returns the value if the type guard passes, otherwise returns the second argument's value
  • The reversed versions
  • Generator provided for custom type guards for non-primitive types

Installation

npm install type-assurer

Usage

The library provides 8 utility functions for each type guard, such as isString, isNull, etc.

And, note that fallbackNotNil can be replaced with the ?? operator. Functions that can be simplified using standard JavaScript expressions, like this example, are not targeted for implementation.

is, isNot

Functions such as is simply provide type guards that can be used in conditional branches.

import { isString } from 'type-assurer'

declare const value: unknown

if (isString(value)) {
  console.log(`This is a string: ${value}`)
} else {
  console.log('This is not a string')
}

Functions such as isNot are useful in cases that require a type guard function as an argument, such as Array.prototype.filter.

import { isNotNil } from 'type-assurer'

declare const values: string | null

const result = values.filter(isNotNil)
//    ^? string[]

assert, assertNot

Functions with names like assert assertNot are type assertion functions. If the type check does not pass, it throws a TypeError.

The second argument can contain an error message.

import { assertString } from 'type-assurer'

declare const value: unknown

assertString(value, 'Value must be a string')
// No error if value is a string, otherwise throws an error with the message "Value must be a string"

ensure, ensureNot

Functions with names like ensure ensureNot are type assertion functions, but return the same value if the type check passes. It is convenient to write type assertions on a single line.

The second argument can contain an error message.

import { ensureString } from 'type-assurer'

declare function fetchData(): Promise<string | undefined>

const value = ensureString(await fetchData(), 'Value must be a string')
//    ^? string
// No error if fetchData returns a string, otherwise throws an error with the message "Value must be a string"

fallback, fallbackNot

Functions like fallback fallbackNot are type modification functions.

They return the same value if the type check passes, otherwise they return the fallback value specified in the second argument.

import { fallbackString } from 'type-assurer'

declare function fetchData(): Promise<string | undefined>

const value = fallbackString(await fetchData(), 'default')
//    ^? string
// Returns value if it's a string, otherwise returns the fallbackValue

Contributing

Contributions are welcome! Please submit a pull request or open an issue to discuss any proposed changes or feature requests.

License

MIT