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.

JS_NewContext

Creates a new JavaScript context with all standard intrinsic objects.

JSContext *JS_NewContext(JSRuntime *rt);

JSRuntime*

required

The runtime in which to create the context

Returns: A new JSContext pointer, or NULL on failure.

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.

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);

JS_FreeContext

Frees a JavaScript context and releases all associated resources.

void JS_FreeContext(JSContext *ctx);

JSContext*

required

The context to free

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.

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

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);

JS_DupContext

Creates a new context that shares the same runtime and global objects.

JSContext *JS_DupContext(JSContext *ctx);

JSContext*

required

The context to duplicate

Returns: A new JSContext pointer that shares objects with the original context, or NULL on failure.

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.

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

Runtime Management

Context Management

Value Operations

Function Calls

Object Operations

Script Evaluation

Exception Handling

Atoms

Context Creation

Context Creation

JS_NewContext

Description

Example

JS_FreeContext

Description

Important Notes

Example

JS_DupContext

Description

Example

​Context Creation

​JS_NewContext

​Description

​Example

​JS_FreeContext

​Description

​Important Notes

​Example

​JS_DupContext

​Description

​Example

​Related Functions

Context Creation

JS_NewContext

Description

Example

JS_FreeContext

Description

Important Notes

Example

JS_DupContext

Description

Example

Related Functions