// Tutorial //

Flow Generic Constraints

Published on July 6, 2017
    Default avatar
    By Matthew Garcia
    Developer and author at DigitalOcean.
    Flow Generic Constraints

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    There are cases in Flow where you’ll want to create a generic function or class, but want to limit what can be passed as a type argument.

    Where Generic Constraints are Useful

    Let’s say you have a class that wraps a Map, but makes a shallow copy before inserting:

    class ShallowMap<T> {
      map: Map<string, T>;
    
      constructor() {
        this.map = new Map();
      }
    
      get(key: string) {
        return this.map.get(key);
      }
    
      set(key: string, value: T) {
        // Make a shallow copy.  Flow will give an error on this line.
        const copy = {...value};
        this.map.set(key, copy);
      }
    }
    

    Flow won’t let you do this, since, while objects can be spread, other types can’t:

    // This works.
    const spreadEmptyObject = {...Object.create(null)};
    const spreadObjectWithProperty = {...{id: 2}};
    // This doesn't.
    const spreadNumber = {...0};
    const spreadString = {...''};
    

    And there’s nothing saying T has to be an object.

    Adding Something that Says `T` Has to Be An Object

    It’s pretty simple; just add a colon and the constraining type after the generic type declaration:

    // `T` has to be an object.
    class ShallowMap<T: Object> {
      map: Map<string, T>;
    
      constructor() {
        this.map = new Map();
      }
    
      get(key: string) {
        return this.map.get(key);
      }
    
      set(key: string, value: T) {
        // Flow is okay with this line now.
        const copy = {...value};
        this.map.set(key, copy);
      }
    }
    

    More Specific Constraints

    Constraints can be more specific, allowing more complex interactions with generic types, since they fit some criteria. For example, a generic map that uses a property of T to determine the key:

    type Keyable = {
      key: string,
    };
    
    // `T` has a string property called `key`.
    class AutoKeyMap<T: {key: string}> {
      map: Map<string, T>;
    
      constructor() {
        this.map = new Map();
      }
    
      get(key: string) {
        return this.map.get(key);
      }
    
      set(value: T) {
        // Since `T` has a string property `key`, we can access it.
        const key = value.key;
        this.map.set(key, value);
      }
    }
    

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    About the authors
    Default avatar
    Developer and author at DigitalOcean.

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