globalThis - JavaScript 编辑

The global globalThis property contains the global this value, which is akin to the global object.

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Description

Historically, accessing the global object has required different syntax in different JavaScript environments. On the web you can use window, self, or frames - but in Web Workers only self will work. In Node.js none of these work, and you must instead use global.
The this keyword could be used inside functions running in non–strict mode, but this will be undefined in Modules and inside functions running in strict mode. You can also use Function('return this')(), but environments that disable eval(), like CSP in browsers, prevent use of Function in this way.

The globalThis property provides a standard way of accessing the global this value (and hence the global object itself) across environments. Unlike similar properties such as window and self, it's guaranteed to work in window and non-window contexts. In this way, you can access the global object in a consistent manner without having to know which environment the code is being run in. To help you remember the name, just remember that in global scope the this value is globalThis.

HTML and the WindowProxy

In many engines globalThis will be a reference to the actual global object, but in web browsers, due to iframe and cross-window security considerations, it references a Proxy around the actual global object (which you can't directly access). This distinction is rarely relevant in common usage, but important to be aware of.

Naming

Several other popular name choices such as self and global were removed from consideration because of their potential to break compatibility with existing code. See the language proposal's "naming" document for more details.

Examples

Search for the global across environments

Prior to globalThis, the only reliable cross-platform way to get the global object for an environment was Function('return this')(). However, this causes CSP violations in some settings, so es6-shim uses a check like this, for example:

var getGlobal = function () {
  if (typeof self !== 'undefined') { return self; }
  if (typeof window !== 'undefined') { return window; }
  if (typeof global !== 'undefined') { return global; }
  throw new Error('unable to locate global object');
};

var globals = getGlobal();

if (typeof globals.setTimeout !== 'function') {
  // no setTimeout in this environment!
}

With globalThis available, the additional search for the global across environments is not necessary anymore:

if (typeof globalThis.setTimeout !== 'function') {
  // no setTimeout in this environment!
}

Specifications

Browser compatibility

BCD tables only load in the browser

Implementation progress

The following table provides a daily implementation status for this feature, because this feature has not yet reached cross-browser stability. The data is generated by running the relevant feature tests in Test262, the standard test suite of JavaScript, in the nightly build, or latest release of each browser's JavaScript engine.

See also

• this