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    4.0.0-alpha • Public • Published


    Extensible, Chainable Functional Javascript Data Validation.

    Npm Version GitHub license

    A validation library design to be a light weight, agnostic, set of 'pure' functional lego bricks to build user input validation fro. fn-validate supports tree shaking and adheres to sementic versioning.



    The simpliest way to install in Node is:

    npm i fn-validate --save
    # Or 
    yarn add fn-validate

    In the browser:

    <script src="" crossorigin="anonymous"></script>
        // fnValidate global available

    Bundle Details

    `fn-validate` comes bundled in three flavours: ES Modules, IIFE and CJS.

    The ES Modules build is compiled to run in any browser supports modules e.g. <script type="module"></script>. Because those browsers support many modern features (e.g. async/await, arrow functions, Map, Set, etc), the output is a lot cleaner and in theory a lot quicker as it runs builtin features rather than polyfilled versions.

    IIFE (immediately invoked function expression) is a self executing function that exposes an fnValidate global designed for using directly in the browser and compiled to run in anything above IE 11.

    CJS (CommonJS) is also compiled to run in older Node versions and when imported via require() statement. It is also compiled down to a version that will run in anything above IE 11.

    See Browser Support for using it with bundlers.

    NB: As can be seen in package.json this project supports several entry point fields: main, module and exports.

    main is for older versions of NodeJS, but also the exports field for serving the ES Modules version if imported or CJS if required for more modern NodeJS versions. It also supports the module field for use with bundlers and supports

    Finally you can use the ES version of the code directly:

    <script type="module" crossorigin="anonymous">
        import { required } from '';
    <script src="" nomodule crossorigin="anonymous"></script>
    <script nomodule>

    Browser Support

    • IIFE = IE11+
    • CJS: NodeJS & IE11+
    • ESModules: Browsers that support ES Modules

    Browser Support with a bundler (e.g. webpack/rollup)

    When using a bundler they will use the ES Modules version by default as its exposed via the module field. This is desireable as it will enable treeshaking and also use clean/ modern code, but if you need to support older browsers you will need to include it in babel's compliation. Most devs exclude code in node_modules to speed compliation so change the exclude field to: exclude: /node_modules(?!\/(fn-validate))/

    What is a Validator?

    A validator is way of validating data, it is a function that accepts data as a single parameter and returns an array stating whether the data was valid or not. If the array is empty, the data was valid, if not valid the array contains a descriptive error message.

    validator('This is a long message'); // => ['Too long']

    So what does this library provide? It is a collection of validator 'creator' functions (higher order functions) that create different types of validators and importantly a way to combine them.

    import { maxLength } from 'fn-validate';
    // create a reuseable validator to check that a string is not longer than 5 characters long
    const validator = maxLength(5);
    // use the validator to check inputs from user input
    validator('Hi'); // => []
    validator('hello'); // => []
    validator('Hello world'); // ['Too long']

    Combining Validators

    On its own that would be simple to implement by hand, but say we also need the value to be required and also an email address as well?

    import { email, maxLength, required, combine } from 'fn-validate';
    // Create a validator that checks the value for multiple things using the combine method.
    const validator = combine([
    validator('') // => []
    validator(''); // => ['Required']
    validator('asdasdasd'); // ['Email address is not valid']
    validator(''); // => ['Too long']

    By default combine returns the first error found, however sometimes only showing one error at a time can be a bad user experience. To return all validation errors, pass true as the second argument.

    import { email, maxLength, required, combine } from 'fn-validate';
    // Pass `true` as the second parameter, to return all validation errors.
    const validator = combine([maxLength(20), required(), email()], true);
    validator('this is a long message that isnt an email address');
    // => ['Too long', 'Email address is not valid']

    Best of all combine simply returns a new validator, so it can be combined with other validators in the same way :D

    Creating Custom Validators

    A validator is just a function that returns an array with an error or not. So creating a custom validator to combine with others is very simple.

    const saysHello = (val) => {
        return val === 'hello' ? [] : [`Input should say 'hello'`];

    This can then be combined with other validators using combine.

    Asynchronous Validators

    Sometimes you need to validate against a server, or another asynchronous action. To do that we use the combineAsync helper instead of combine which returns a Promise.


    Below we check a username is available on a server, by creating an async validator that sends a request to the server and using it with combineAsync. NB: An async validator is only different to a normal validator as it returns a Promise that resolves to the array, rather than just an array.

    /* checkUsernameAvailable.js */
    export const checkUsernameAvailable = async (username) => {
        const url = `https://some.api/username/${encodeURIComponent(username)}`;
        const response = await fetch(url);
        if(!response.ok) {
            throw new Error('Error checking the username');
        const result = await response.text();
        return data === 'true' ? [] : ['Username is not available'];
    /* index.js */
    import { required, combineAsync } from 'fn-validate';
    import { checkUsernameAvailable } from './checkUsernameAvailable';
    const validator = combineAsync([
    validator(''); // => Promise resolved to ['Required']
    validator('existing_username'); // => Promise resolved to ['Username is not available']
    validator('an_unused_username'); // Promise resolved to []

    Perfomrance Tip: Place any synchronous validators first as validators are called in sequence. If a validator returns a synchronous result that isnt valid, it will return immediately without calling later validators. Obviously this tip doest work with runAll set to true.

    Pre-defined Validators

    Below are the validators supplied with fn-validate, NB: fn-validate supports tree shaking, so if using a build tool like webpack or rollup you wont include functions in your build that you dont use.

    Validator Description
    required If a value is 'falsy' it is considered invalid
    notDefault Similar to required, checks a value against a default or list of default values to en**sure its been changed.
    isNumeric Checks if a value is a number or whether it can be parsed as a number (via parseFloat)
    email Checks a string against a regex to see if the value matches an email address format or not
    maxLength Checks a string is not longer longer than the number stated
    minLength Checks a string is than the number stated
    lengthBetween Checks a string is between a minimum and maximum length (inclusive)
    matches Checks a value matches a regexp pattern
    equalTo Compares to a value to a another value or the value returned from a function
    notEqualTo Opposite of equalTo
    hasChar Checks a value has at least one of the characters supplied. Useful for password va**lidation
    hasLowercase Checks a value has at least one lowercase character. Useful for password validation
    hasNumeric Checks a value has at least one numeric character. Useful for password validation
    hasUppercase Checks a value has at least one numeric character. Useful for password validation
    between Checks a number is between a minimum and maximum size (inclusive)
    lessThan Checks value is less than a supplied value
    lessThanOrEqualTo Checks value is less than or equal to a supplied value
    greaterThan Ensures a value is more than a supplied value
    greaterThanOrEqualTo Checks value is greater than or equal to a supplied value

    Overriding Error Messages

    Each of the pre-defined validators has a default message, but its possible to change the error message of a validator when creating it. Which is important for localis(z)ation.

    let isRequired = required('A new error message');
    let moreThan10 = moreThan('Woooo that value is way too small');
    isRequired(null); // ['A new error message']
    moreThan10(5); // == ['Woooo that value is way too small']


    • Add specific validators to the library now its been cleaned up E.g. isCreditCardNo etc as I come across a need for them.
    • Convert to Typescript.


    yarn run build


    Remember to login first, includes pre-testing stage.

    yarn run deploy


    npm i fn-validate

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