// Tutorial //

Parameter Variance in Flow

Published on July 4, 2017
    Default avatar
    By Matthew Garcia
    Developer and author at DigitalOcean.
    Parameter Variance in Flow

    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.

    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;

    Want to learn more? Join the DigitalOcean Community!

    Join our DigitalOcean community of over a million developers for free! Get help and share knowledge in our Questions & Answers section, find tutorials and tools that will help you grow as a developer and scale your project or business, and subscribe to topics of interest.

    Sign up
    About the authors
    Default avatar
    Developer and author at DigitalOcean.

    Still looking for an answer?

    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!