Tutorial

Parameter Variance in Flow

Published on July 4, 2017
    author

    By Matthew Garcia

    Parameter Variance in Flow

    There are times when you may want a function to be able to take different types of parameters. A little bit like union types in TypeScript. Let’s see how it can be done using Flow.

    Creating a Function That Takes a Parameter of Multiple Types

    It’s just a matter of listing the possible types, delimited by a |:

    function capitalize(
      // `capitalize` can take a string or an array of strings.
      words: string | Array<string>,
      // `capitalize` will return an array of strings either way.
    ): Array<string> {
      // At this point, Flow does not know if `words` is a string or an array,
      // so properties or methods specific to those types,
      // such as `map` and `toUpperCase`, cannot be accessed.
      // However, properties or methods that are shared by both, such as `length`, can be accessed.
      if (!Array.isArray(words)) {
        // Flow now knows that `words` is a string, so we can use string methods.
        return [words.toUpperCase()];
      }
      // Flow now knows that `words` is an array, so we can use array methods.
      return words.map(word => word.toUpperCase());
    }
    

    Helping Flow Determine the Type

    The above example used Array.isArray to determine the type of words, but there are other ways Flow can do this:

    // Through truthiness
    function cube(val: number | void): number {
      // If `val` is falsy, it must be undefined (or 0).
      if (!val) {
        return 0;
      }
      // Otherwise, `val` must be a number.
      return val ** 3;
    }
    
    // With typeof
    function getDigits(val: string | number): number {
      // If typeof val is 'number', it must be a number.
      if (typeof val === 'number') {
        return Math.ceil(Math.log10(val));
      }
      // Otherwise,`val` must be a string.
      return val.length;
    }
    
    // With instanceof
    function getName(name: string | Nametag): string {
      // If `name` is an instance of the class `Nametag`, it must be a Nametag.
      if (name instanceof Nametag) {
        return name.name;
      }
      // Otherwise, `name` must be a string.
      return name;
    }
    

    Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.

    Learn more about our products

    About the author(s)

    Category:
    Tutorial
    Tags:

    While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

    Still looking for an answer?

    Ask a questionSearch for more help

    Was this helpful?
     
    Leave a comment
    

    This textbox defaults to using Markdown to format your answer.

    You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!

    Join the Tech Talk
    Success! Thank you! Please check your email for further details.

    Please complete your information!

    Become a contributor for community

    Get paid to write technical tutorials and select a tech-focused charity to receive a matching donation.

    DigitalOcean Documentation

    Full documentation for every DigitalOcean product.

    Resources for startups and SMBs

    The Wave has everything you need to know about building a business, from raising funding to marketing your product.

    Get our newsletter

    Stay up to date by signing up for DigitalOcean’s Infrastructure as a Newsletter.

    New accounts only. By submitting your email you agree to our Privacy Policy

    The developer cloud

    Scale up as you grow — whether you're running one virtual machine or ten thousand.

    Get started for free

    Sign up and get $200 in credit for your first 60 days with DigitalOcean.*

    *This promotional offer applies to new accounts only.