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PR-URL: https://github.com/nodejs/node/pull/32676 Reviewed-By: Rich Trott <rtrott@gmail.com>
72 lines
2.3 KiB
Markdown
72 lines
2.3 KiB
Markdown
# Using global symbols
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ES6 introduced a new type: `Symbol`. This new type is _immutable_, and
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it is often used for metaprogramming purposes, as it can be used as
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property keys like string. There are two types of
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symbols, local and global.
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Symbol-keyed properties of an object are not included in the output of
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`JSON.stringify()`, but the `util.inspect()` function includes them by
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default.
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Learn more about symbols at
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<https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Symbol>.
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## `Symbol(string)`
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Symbols created via `Symbol(string)` are local to the caller function.
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For this reason, we often use them to simulate private fields, like so:
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```js
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const kField = Symbol('kField');
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console.log(kField === Symbol('kField')); // false
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class MyObject {
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constructor() {
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this[kField] = 'something';
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}
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}
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module.exports.MyObject = MyObject;
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```
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Symbols are not fully private, as the data could be accessed anyway:
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```js
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for (const s of Object.getOwnPropertySymbols(obj)) {
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const desc = s.toString().replace(/Symbol\((.*)\)$/, '$1');
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if (desc === 'kField') {
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console.log(obj[s]); // 'something'
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}
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}
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```
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Local symbols make it harder for developers to monkey patch/access
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private fields, as they require more work than a property prefixed
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with an `_`. Monkey patching private API that were not designed to be
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monkey-patchable make maintaining and evolving Node.js harder, as private
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properties are not documented and can change within a patch release.
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Some extremely popular modules in the ecosystem monkey patch some
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internals, making it impossible for us to update and improve those
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areas without causing issues for a significant amount of users.
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## `Symbol.for`
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Symbols created with `Symbol.for(string)` are global and unique to the
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same V8 Isolate. On the first call to `Symbol.for(string)` a symbol is
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stored in a global registry and easily retrieved for every call of
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`Symbol.for(string)`. However, this might cause problems when two module
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authors use the same symbol
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for different reasons.
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```js
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const s = Symbol.for('hello');
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console.log(s === Symbol.for('hello'));
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```
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In the Node.js runtime we prefix all our global symbols with `nodejs.`,
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e.g. `Symbol.for('nodejs.hello')`.
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Global symbols should be preferred when a developer-facing interface is
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needed to allow behavior customization, i.e., metaprogramming.
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