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Create a new JSString whose characters are stored in external memory.


JSString *
JS_NewExternalString(JSContext *cx, const char16_t *chars, size_t length,
                     const JSStringFinalizer *fin);

JSString *
JS_NewExternalStringWithClosure(JSContext *cx, jschar *chars, size_t length,
                                int type, void *closure); // Obsolete since JSAPI 13
Name Type Description
cx JSContext * The context in which to create the new string. Requires request. In a JS_THREADSAFE build, the caller must be in a request on this JSContext.
chars const char16_t * or const jschar * Pointer to the first element of an array of char16_ts. This array is used as the character buffer of the JSString to be created. The array must be populated with the desired character data before JS_NewExternalString is called, and the array must remain in memory, with its contents unchanged, for as long as the JavaScript engine needs to hold on to it. (Ultimately, the string will be garbage collected, and the JavaScript engine will call the string finalizer callback, allowing the application to free the array.) The array does not need to be zero-terminated.
length size_t The number of characters in the array chars.
fin JSStringFinalizer A string finalizer the JavaScript engine should use (later) to free the string buffer chars. Added in SpiderMonkey 17
type int (JS_NewExternalStringWithClosure only) Indicates which string finalizer callback the JavaScript engine should use (later) to free the string buffer chars. This must be an id number previously returned by a successful call to JS_AddExternalStringFinalizer. Obsolete since JSAPI 13
closure void * (JS_NewExternalStringWithClosure only) Arbitrary, application-defined data to include in the string object after it's created. If you use this as a pointer, you should clean it up in the external string finalizer that was previously set up using JS_AddExternalStringFinalizer. Obsolete since JSAPI 13


JS_NewExternalString and JS_NewExternalStringWithClosure create a new JSString whose characters are stored in external memory, i.e., memory allocated by the application, not the JavaScript engine. Since the program allocated the memory, it will need to free it; this happens in an external string finalizer indicated by the type parameter.

JS_NewExternalStringWithClosure works similarly, except it accepts an additional parameter, closure, which is saved in the string's internal data structure. It can be retrieved later by calling JS_GetExternalStringClosure. This is application-defined data you can associate with the string.

To determine if a string was created as an external string, you can call JS_IsExternalString.

See Also

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