HTTP/2 301
server: nginx
date: Wed, 23 Jul 2025 05:18:07 GMT
content-type: text/html
content-length: 162
location: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/keyFor
strict-transport-security: max-age=60; includeSubDomains
via: 1.1 google
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000
HTTP/2 200
content-type: text/html
x-guploader-uploadid: ABgVH8_-iFLnvtKCct8wVpzh2K0E4-9_36SwvS0pm3dfqzigpC_PDCQ0vn1zpA3PU5vWeYt0
cache-control: public, max-age=3600
expires: Wed, 23 Jul 2025 06:18:08 GMT
last-modified: Wed, 23 Jul 2025 00:48:08 GMT
etag: W/"059c44bb6e1807ea75cb4977c6f4dc6d"
x-goog-generation: 1753231688290001
x-goog-metageneration: 1
x-goog-stored-content-encoding: identity
x-goog-stored-content-length: 68385
x-goog-meta-goog-reserved-file-mtime: 1753230413
x-goog-hash: crc32c=ViYDRQ==, md5=BZxEu24YB+p1y0l3xvTcbQ==
x-goog-storage-class: STANDARD
accept-ranges: none
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000
alt-svc: clear
x-content-type-options: nosniff
strict-transport-security: max-age=63072000
content-security-policy: default-src 'self'; script-src 'report-sample' 'self' 'wasm-unsafe-eval' https://www.google-analytics.com/analytics.js https://www.googletagmanager.com/gtag/js assets.codepen.io production-assets.codepen.io https://js.stripe.com 'sha256-EehWlTYp7Bqy57gDeQttaWKp0ukTTEUKGP44h8GVeik=' 'sha256-XNBp89FG76amD8BqrJzyflxOF9PaWPqPqvJfKZPCv7M='; script-src-elem 'report-sample' 'self' 'wasm-unsafe-eval' https://www.google-analytics.com/analytics.js https://www.googletagmanager.com/gtag/js assets.codepen.io production-assets.codepen.io https://js.stripe.com 'sha256-EehWlTYp7Bqy57gDeQttaWKp0ukTTEUKGP44h8GVeik=' 'sha256-XNBp89FG76amD8BqrJzyflxOF9PaWPqPqvJfKZPCv7M='; style-src 'report-sample' 'self' 'unsafe-inline'; object-src 'none'; base-uri 'self'; connect-src 'self' developer.allizom.org bcd.developer.allizom.org bcd.developer.mozilla.org updates.developer.allizom.org updates.developer.mozilla.org https://*.google-analytics.com https://*.analytics.google.com https://*.googletagmanager.com https://incoming.telemetry.mozilla.org https://observatory-api.mdn.allizom.net https://observatory-api.mdn.mozilla.net https://api.github.com/search/issues stats.g.doubleclick.net https://api.stripe.com; font-src 'self'; frame-src 'self' interactive-examples.mdn.mozilla.net interactive-examples.mdn.allizom.net mdn.github.io live-samples.mdn.mozilla.net live-samples.mdn.allizom.net *.mdnplay.dev *.mdnyalp.dev *.play.test.mdn.allizom.net https://v2.scrimba.com https://scrimba.com jsfiddle.net www.youtube-nocookie.com codepen.io survey.alchemer.com https://js.stripe.com; img-src 'self' data: *.githubusercontent.com *.googleusercontent.com *.gravatar.com mozillausercontent.com firefoxusercontent.com profile.stage.mozaws.net profile.accounts.firefox.com developer.mozilla.org mdn.dev interactive-examples.mdn.mozilla.net interactive-examples.mdn.allizom.net wikipedia.org upload.wikimedia.org https://mdn.github.io/shared-assets/ https://mdn.dev/ https://*.google-analytics.com https://*.googletagmanager.com www.gstatic.com; manifest-src 'self'; media-src 'self' archive.org videos.cdn.mozilla.net https://mdn.github.io/shared-assets/; child-src 'self'; worker-src 'self';
x-frame-options: DENY
origin-trial: AxVILwizhbMjxFeHOn1P3R8niO1RJY/smaK4B4d1rLzc1gTaxtXMSaTi+FoigYgCw40uFRDwFcEAeqDR+vVLOW4AAABfeyJvcmlnaW4iOiJodHRwczovL2RldmVsb3Blci5tb3ppbGxhLm9yZyIsImZlYXR1cmUiOiJQcml2YXRlQXR0cmlidXRpb25WMiIsImV4cGlyeSI6MTc0MjA3OTYwMH0=
x-cloud-trace-context: c0c65f35f6eaa196dcfa2f743674f804
date: Wed, 23 Jul 2025 05:18:08 GMT
server: Google Frontend
via: 1.1 google
vary: Accept-Encoding
content-encoding: gzip
x-cache: miss
Symbol.keyFor() - JavaScript | MDN
Symbol.keyFor() Baseline Widely available
The Symbol.keyFor()
static method retrieves a shared symbol
key from the global symbol registry for the given symbol.
const globalSym = Symbol.for("foo"); // Global symbol
console.log(Symbol.keyFor(globalSym));
// Expected output: "foo"
const localSym = Symbol(); // Local symbol
console.log(Symbol.keyFor(localSym));
// Expected output: undefined
console.log(Symbol.keyFor(Symbol.iterator));
// Expected output: undefined
sym
Symbol, required. The symbol to find a key for.
const globalSym = Symbol.for("foo"); // create a new global symbol
Symbol.keyFor(globalSym); // "foo"
const localSym = Symbol();
Symbol.keyFor(localSym); // undefined
// well-known symbols are not symbols registered
// in the global symbol registry
Symbol.keyFor(Symbol.iterator); // undefined