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Context Creation

These functions manage the lifecycle of JavaScript contexts (JSContext). A context represents a JavaScript realm with its own global objects and system objects. Multiple contexts can exist within a single runtime and share objects, similar to how frames of the same origin share JavaScript objects in a web browser.

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JS_NewContext

Creates a new JavaScript context with all standard intrinsic objects. 
JSContext *JS_NewContext(JSRuntime *rt);
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rt
JSRuntime*
required
The runtime in which to create the context
Returns: A new JSContext pointer, or NULL on failure.

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Description

This function creates a fully-featured JavaScript context with all standard intrinsic objects including:
  • Base objects (Object, Function, Array, etc.)
  • Date
  • Eval
  • RegExp
  • JSON
  • Proxy
  • Map and Set
  • TypedArrays
  • Promise
  • BigInt
For memory-constrained environments, use JS_NewContextRaw() combined with selective intrinsic functions to create a minimal context.

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Example

// Create a runtime
JSRuntime *rt = JS_NewRuntime();
if (!rt) {
    fprintf(stderr, "Failed to create runtime\n");
    return 1;
}

// Create a context with all standard objects
JSContext *ctx = JS_NewContext(rt);
if (!ctx) {
    fprintf(stderr, "Failed to create context\n");
    JS_FreeRuntime(rt);
    return 1;
}

// Use the context...
JSValue global = JS_GetGlobalObject(ctx);
JS_FreeValue(ctx, global);

// Cleanup
JS_FreeContext(ctx);
JS_FreeRuntime(rt);

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JS_FreeContext

Frees a JavaScript context and releases all associated resources. 
void JS_FreeContext(JSContext *ctx);
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ctx
JSContext*
required
The context to free

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Description

This function releases all resources associated with the context, including:
  • All JavaScript objects created in the context
  • The global object and its properties
  • All pending exceptions
  • Internal data structures
You must free all contexts before freeing the runtime. Failing to do so will result in memory leaks.

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Important Notes

  • Always call JS_FreeContext() before JS_FreeRuntime()
  • All JSValue objects created in this context should be freed before calling this function
  • Do not use the context pointer after calling this function

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Example

JSRuntime *rt = JS_NewRuntime();
JSContext *ctx1 = JS_NewContext(rt);
JSContext *ctx2 = JS_NewContext(rt);

// Use contexts...

// Free contexts before runtime
JS_FreeContext(ctx1);
JS_FreeContext(ctx2);
JS_FreeRuntime(rt);

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JS_DupContext

Creates a new context that shares the same runtime and global objects. 
JSContext *JS_DupContext(JSContext *ctx);
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ctx
JSContext*
required
The context to duplicate
Returns: A new JSContext pointer that shares objects with the original context, or NULL on failure.

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Description

This function creates a new context within the same runtime. The new context:
  • Shares the same global object as the original context
  • Can access and modify objects from the original context
  • Behaves like the same JavaScript realm
  • Must be freed separately with JS_FreeContext()
This is useful for scenarios where you need multiple references to the same realm, such as implementing worker threads or isolating execution while sharing state.

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Example

JSRuntime *rt = JS_NewRuntime();
JSContext *ctx = JS_NewContext(rt);

// Set a global variable in the original context
JSValue global = JS_GetGlobalObject(ctx);
JS_SetPropertyStr(ctx, global, "shared", JS_NewInt32(ctx, 42));
JS_FreeValue(ctx, global);

// Duplicate the context
JSContext *ctx2 = JS_DupContext(ctx);

// Access the shared variable from the duplicated context
JSValue result = JS_Eval(ctx2, "shared", 6, "<eval>", JS_EVAL_TYPE_GLOBAL);
// result will be 42

int32_t value;
JS_ToInt32(ctx2, &value, result);
printf("Shared value: %d\n", value); // Prints: Shared value: 42

JS_FreeValue(ctx2, result);

// Free both contexts
JS_FreeContext(ctx2);
JS_FreeContext(ctx);
JS_FreeRuntime(rt);
  • JS_NewContextRaw() - Create a minimal context without intrinsic objects
  • JS_GetRuntime() - Get the runtime associated with a context
  • JS_GetContextOpaque() / JS_SetContextOpaque() - Attach custom data to a context
  • JS_GetGlobalObject() - Get the global object for a context

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