Zach Snoek
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Understanding Symbols in JavaScript

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Zach Snoek
·May 3, 2022·

6 min read

Featured on Hashnode

What are symbols?

Symbols are primitive values that are guaranteed to be unique. You can create a new symbol with the Symbol constructor and optionally provide a description string:

const foo = Symbol('foo');

Symbols are primitives, not objects. We can verify this by checking the typeof a symbol:

const foo = Symbol('foo');

console.log(typeof foo); // "symbol"

console.log(foo instanceof Object); // false

However, because symbols are unique, they act like objects when checking for equality. Two different symbols will never be equal, which is not true for other primitive values:

// Symbols are unique, like object references
console.log(Symbol() === Symbol()); // false
console.log({} === {}); // false

// Other primitives are not unique; their contents are equal
console.log('foo' === 'foo'); // true
console.log(42 === 42); // true

// The same symbol is equal to itself
const foo = Symbol();
console.log(foo === foo); // true

Symbols can also be used as property keys in objects; we call these property symbols. Property symbols are accessed via bracket notation:

const foo = Symbol();

const bar = {
    [foo]: 'foo',
}

console.log(bar[foo]); // "foo"

Note that property symbols won’t appear in for...of loops or get returned from Object.getOwnPropertyNames. However, they’re not completely private: we can use Object.getOwnPropertySymbols or Reflect.ownKeys to retrieve them:

const fooSymbol = Symbol('foo');

const foo = {
    [fooSymbol]: 'foo symbol',
    fooString: 'foo string',
};

// Only "fooString" is logged
for (const key in foo) {
    console.log(key); // "fooString"
}

// Object.getOwnPropertySymbols returns an array of property symbols: [fooSymbol]
console.log(Object.getOwnPropertySymbols(foo).length); // 1
console.log(Object.getOwnPropertySymbols(foo)[0]); // Symbol(foo)

The global symbol registry

In our previous examples, we created symbols as local variables. However, it might be necessary for a symbol to be available globally or across realms. We can store global symbols in the global symbol registry; to create a global symbol, use Symbol.for:

const foo = Symbol.for('foo');

console.log(foo === Symbol.for('foo')); // true

If a symbol with the given key argument (which is also used as the symbol’s description) does not already exist in the global symbol registry, Symbol.for creates a new symbol and adds it to the registry. Otherwise, it returns the symbol for the key.

If you need to retrieve the key of a global symbol, use Symbol.keyFor:

const foo = Symbol.for('foo');

console.log(Symbol.keyFor(foo)); // "foo"

const bar = Symbol('bar'); // local symbols are not added to the global symbol registry

console.log(Symbol.keyFor(bar)); // undefined

Now that we understand the basics of symbols, let’s explore some use cases.

Use cases

Preventing property key collisions

Symbols are useful for creating unique property keys that won’t clash with other string or symbol keys.

For example, consider the contrived example of a music library that exports a createSong function for creating song objects. Each song returned from createSong has an id used internally by the library:

import { createSong } from 'foo-song-library';

const song = createSong({ title: 'Africa', artist: 'Toto' });

console.log(song); // { id: 42, title: 'Africa', artist: 'Toto' }

Since id is a normal string key, a consumer could easily overwrite it––either intentionally or on accident:

const song = createSong({ title: 'Africa', artist: 'Toto' });

console.log(song.id); // 42

song.id = 'foo';

console.log(song.id); // "foo"

createSong could instead use a property symbol so that it’s harder for the consumer to overwrite:

// foo-song-library

const id = Symbol('id');

function createSong({ title, artist }) {
    return {
        [id]: createUniqueId(),
        title,
        artist,
    }    
}

// Consumer

const song = createSong({ title: 'Africa', artist: 'Toto' });

song.id = 'foo';

console.log(song); // { [Symbol(id)]: 42, id: 'foo', ... }

As we showed earlier, it’s still possible to find the symbol keys of an object, so the internal [id] property isn’t truly inaccessible.

Checking the validity of React elements with unique tags

For this use case, let’s explore a real-world example.

We typically create React elements using JSX. Under the hood, our markup gets transformed to a React.createElement call, which returns a React element––a normal JavaScript object:

// This JSX:

<div
    style={{
      width: '50px',
      height: '50px',
      background: 'peachpuff'
    }}
/>

// Gets transpiled to this:

React.createElement('div', {
  style: {
    width: '50px',
    height: '50px',
    background: 'peachpuff'
  }
});

// React.createElement returns an object that looks like:

{
    type: 'div',
    props: {
        style: {
        width: '50px',
        height: '50px',
        background: 'peachpuff'
      }
    },
    $$typeof: Symbol.for('react.element'),
    // ...
}

In most cases, creating elements like this isn’t useful. But notice that the element created by React.createElement has a $$typeof property whose value is a symbol. React tags elements it creates with this property to ensure that the object it is rendering is valid.

Suppose we’re fetching user data from a server and expecting to render their bio as a string. If our server can mistakenly store JSON in the bio field, we could get back a React element in JSON with some malicious dangerouslySetInnerHTML:

function App() {
    const [user, setUser] = React.useState();

    // Fetch data, etc.

    // user:
    /*
        {
            name: ...,
            bio: {
                type: 'div',
                props: {
                    dangerouslySetInnerHTML: {
                        __html: ...
                    }
                }
                ...
            }
        }
    */

    return (
        <div>
            <h1>Welcome, {user.name}!</h1>
            <p>{user.bio}</p> {/* `bio` is a `div`, not a string! */}
        </div>
    );
}

React tries to prevent this from happening by checking to see if the element it is rendering has a $$typeof property value equal to Symbol.for('react.element'); if it doesn’t, it won’t render the element. This works because JSON can’t have symbol values, so we can’t return a symbol equal to Symbol.for('react.element') from a server.

For a more detailed explanation, see this blog post by Dan Abramov and the PR that added this $$typeof symbol check.

Other characteristics of symbols

String conversion

Converting a symbol to a string can be useful for debugging and other output, but be aware that symbols can’t be implicitly converted to strings. If you want to convert a symbol to a string, use the String primitive wrapper or .toString:

const foo = Symbol('foo');

console.log(foo + ''); // Uncaught TypeError: Cannot convert a Symbol value to a string

console.log(String(foo)); // "Symbol(foo)"

console.log(foo.toString()); // "Symbol(foo)"

You can also use a symbol’s string description by accessing its .description property:

console.log(foo.description); // "foo"

JSON.stringify

Symbol properties will not be returned from JSON.stringify. As we mentioned earlier, symbols are not valid in JSON:

const foo = { [Symbol()]: 'foo', bar: 'bar' };

console.log(JSON.stringify(foo)); // {"bar":"bar"}

Well-known symbols

JavaScript has several well-known symbols. These are symbols that are built into the language and used by algorithms of the specification. We usually encounter them as property keys whose values we implement to extend a part of the specification.

For example, we can make an object iterable by defining a property whose key is Symbol.iterator and whose value is a function that returns a iterator:

const foo = ['foo', 'bar'];

const iterator = foo[Symbol.iterator]();

console.log(iterator.next()); // { value: 'foo', done: false }

A list of all the well-known symbols in the specification can be found here.

Conclusion

If you’re interested in learning more about symbols, I recommend starting with the Symbol API docs on MDN and JavaScript for Impatient Programmers by Dr. Axel Rauschmayer.

That’s all for this post. Good luck!

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