Symbol.prototype.description

Symbol.prototype.description Property

Symbol is a new primitive data type introduced in ECMAScript 6, representing unique values. In ECMAScript 10, the description property was added to Symbol's prototype to retrieve the descriptive string of a Symbol. This property allows developers to more conveniently access a Symbol's description without having to call the toString() method and then process the resulting string.

const sym1 = Symbol('foo');
console.log(sym1.description); // "foo"

const sym2 = Symbol();
console.log(sym2.description); // undefined

Basic Usage of the description Property

The description property is a read-only property that returns the optional descriptive string passed when creating the Symbol. If no description was provided during creation, the description property returns undefined.

// With description
const namedSymbol = Symbol('This is a described Symbol');
console.log(namedSymbol.description); // "This is a described Symbol"

// Without description
const unnamedSymbol = Symbol();
console.log(unnamedSymbol.description); // undefined

Comparison with the toString() Method

Before ECMAScript 10, to obtain a Symbol's description, one typically had to call the toString() method and then extract the description from the returned string:

const oldSym = Symbol('old way');
const desc = oldSym.toString().slice(7, -1);
console.log(desc); // "old way"

With the description property, the code becomes more concise and intuitive:

const newSym = Symbol('new way');
console.log(newSym.description); // "new way"

Practical Application Scenarios

The description property is particularly useful for debugging and logging. For example, when using Symbols as object property keys, description can help identify these Symbols:

const USER_ID = Symbol('user identifier');
const GROUP_ID = Symbol('group identifier');

const user = {
  [USER_ID]: '12345',
  [GROUP_ID]: 'admin'
};

function logUserProperties(user) {
  for (const key of Reflect.ownKeys(user)) {
    if (typeof key === 'symbol') {
      console.log(`Symbol key: ${key.description}, Value: ${user[key]}`);
    } else {
      console.log(`String key: ${key}, Value: ${user[key]}`);
    }
  }
}

logUserProperties(user);
// Output:
// Symbol key: user identifier, Value: 12345
// Symbol key: group identifier, Value: admin

Integration with Other Language Features

The description property works well with other ES6+ features. For example, in template literals:

const STATUS = Symbol('application status');

function getStatusSymbol() {
  return STATUS;
}

console.log(`Current status symbol description: ${getStatusSymbol().description}`);
// Output: "Current status symbol description: application status"

In destructuring assignments:

const { description } = Symbol('destructurable description');
console.log(description); // "destructurable description"

Relationship with Symbol.for() and Symbol.keyFor()

The description property serves a different purpose than Symbol.for() and Symbol.keyFor():

// Symbol.for() uses the global registry
const globalSym = Symbol.for('global symbol');
console.log(globalSym.description); // "global symbol"
console.log(Symbol.keyFor(globalSym)); // "global symbol"

// Regular Symbol
const localSym = Symbol('local symbol');
console.log(localSym.description); // "local symbol"
console.log(Symbol.keyFor(localSym)); // undefined

Application in Class Definitions

In class definitions, Symbols are often used to define private or special methods, where the description property can help identify these methods:

const RENDER = Symbol('render method');

class Component {
  [RENDER]() {
    console.log('Executing render method');
  }

  render() {
    this[RENDER]();
    console.log(`Symbol used: ${RENDER.description}`);
  }
}

const comp = new Component();
comp.render();
// Output:
// Executing render method
// Symbol used: render method

Performance Considerations

Directly accessing the description property is more efficient than calling the toString() method and parsing the string. While this performance difference is negligible in most applications, it may have an impact in high-performance scenarios involving large numbers of Symbols.

const testSym = Symbol('test');

console.time('toString');
for (let i = 0; i < 1000000; i++) {
  const desc = testSym.toString().slice(7, -1);
}
console.timeEnd('toString'); // Approximately 50-100ms

console.time('description');
for (let i = 0; i < 1000000; i++) {
  const desc = testSym.description;
}
console.timeEnd('description'); // Approximately 5-10ms

Browser Compatibility and Transpilation

The description property is widely supported in modern browsers, but for projects requiring support for older browsers, transpilation tools like Babel may be necessary. Babel's @babel/plugin-transform-symbol-proposal plugin ensures the code works correctly in older environments.

Integration with Other New Features

The description property can be combined with ES2017's Object.getOwnPropertySymbols() to conveniently retrieve all Symbol properties of an object and their descriptions:

const OBJ_KEY = Symbol('object key');
const DATA_KEY = Symbol('data key');

const obj = {
  [OBJ_KEY]: 'value1',
  [DATA_KEY]: 'value2',
  regularProp: 'value3'
};

const symbolProps = Object.getOwnPropertySymbols(obj);
symbolProps.forEach(sym => {
  console.log(`Symbol property: ${sym.description}, Value: ${obj[sym]}`);
});
// Output:
// Symbol property: object key, Value: value1
// Symbol property: data key, Value: value2

Application in Iterators and Generators

When using Symbols to define custom iterators, the description property can help identify the iterator's purpose:

class Fibonacci {
  constructor(max = 100) {
    this.max = max;
  }

  [Symbol.iterator]() {
    let [a, b] = [0, 1];
    return {
      next: () => {
        const value = a;
        [a, b] = [b, a + b];
        return { value, done: value > this.max };
      },
      [Symbol.toStringTag]: `Fibonacci Iterator up to ${this.max}`
    };
  }
}

const fib = new Fibonacci(50);
const iterator = fib[Symbol.iterator]();
console.log(iterator[Symbol.toStringTag]); // "Fibonacci Iterator up to 50"

Application in Metaprogramming

The description property is particularly useful in metaprogramming scenarios, such as when using the Proxy or Reflect APIs:

const TRACK = Symbol('property access tracker');

const tracker = {
  count: 0,
  [TRACK](prop) {
    this.count++;
    console.log(`Property ${prop} accessed, total accesses: ${this.count}`);
  }
};

const obj = new Proxy({}, {
  get(target, prop) {
    if (prop in tracker) {
      tracker[TRACK](prop);
    }
    return Reflect.get(target, prop);
  }
});

obj.a = 1;
obj.b = 2;
obj.a; // Triggers tracking
obj.b; // Triggers tracking
console.log(`Tracking Symbol description: ${TRACK.description}`);
// Output:
// Property a accessed, total accesses: 1
// Property b accessed, total accesses: 2
// Tracking Symbol description: property access tracker

Application in Regular Expressions

Although uncommon, Symbols can also be used to extend regular expressions, where the description property can provide useful information:

const REGEX_INFO = Symbol('regex additional info');

RegExp.prototype[REGEX_INFO] = function() {
  return {
    source: this.source,
    flags: this.flags,
    description: REGEX_INFO.description
  };
};

const regex = /test/gi;
console.log(regex[REGEX_INFO]());
// Output:
// {
//   source: "test",
//   flags: "gi",
//   description: "regex additional info"
// }

Application in Error Handling

Using Symbols and description in custom error classes can create richer error messages:

const ERROR_CODE = Symbol('error code symbol');

class CustomError extends Error {
  constructor(message, code) {
    super(message);
    this[ERROR_CODE] = code;
  }

  get codeInfo() {
    return `Error code: ${this[ERROR_CODE]} (${ERROR_CODE.description})`;
  }
}

try {
  throw new CustomError('Custom error occurred', 500);
} catch (e) {
  console.log(e.message); // "Custom error occurred"
  console.log(e.codeInfo); // "Error code: 500 (error code symbol)"
}

Application in Data Structures

When implementing custom data structures, Symbols can serve as internal markers, with the description property aiding in debugging:

const INTERNAL_STATE = Symbol('linked list internal state marker');

class LinkedList {
  constructor() {
    this[INTERNAL_STATE] = {
      head: null,
      size: 0
    };
  }

  get size() {
    return this[INTERNAL_STATE].size;
  }

  add(value) {
    const node = { value, next: null };
    if (!this[INTERNAL_STATE].head) {
      this[INTERNAL_STATE].head = node;
    } else {
      let current = this[INTERNAL_STATE].head;
      while (current.next) {
        current = current.next;
      }
      current.next = node;
    }
    this[INTERNAL_STATE].size++;
  }

  debug() {
    console.log(`Data structure internal state marker: ${INTERNAL_STATE.description}`);
    console.log(`Current size: ${this.size}`);
  }
}

const list = new LinkedList();
list.add(1);
list.add(2);
list.debug();
// Output:
// Data structure internal state marker: linked list internal state marker
// Current size: 2