Parameter Variance in Flow

Published on July 4, 2017
    Default avatar

    By Matthew Garcia

    Parameter Variance in Flow

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    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;

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    About the authors
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    Matthew Garcia


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