# QuickJS Javascript Engine

## Table of Contents

- 1 Introduction
  - 1.1 Main Features
- 2 Usage
  - 2.1 Installation
  - 2.2 Quick start
  - 2.3 Command line options
    - 2.3.1 `qjs` interpreter
    - 2.3.2 `qjsc` compiler
  - 2.4 Built-in tests
  - 2.5 Test262 (ECMAScript Test Suite)
- 3 Specifications
  - 3.1 Language support
    - 3.1.1 ES2025 support
    - 3.1.2 ECMA402
  - 3.2 Modules
  - 3.3 Standard library
    - 3.3.1 Global objects
    - 3.3.2 `std` module
    - 3.3.3 `os` module
  - 3.4 QuickJS C API
    - 3.4.1 Runtime and contexts
    - 3.4.2 JSValue
    - 3.4.3 C functions
    - 3.4.4 Exceptions
    - 3.4.5 Script evaluation
    - 3.4.6 JS Classes
    - 3.4.7 C Modules
    - 3.4.8 Memory handling
    - 3.4.9 Execution timeout and interrupts
- 4 Internals
  - 4.1 Bytecode
  - 4.2 Executable generation
    - 4.2.1 `qjsc` compiler
    - 4.2.2 Binary JSON
  - 4.3 Runtime
    - 4.3.1 Strings
    - 4.3.2 Objects
    - 4.3.3 Atoms
    - 4.3.4 Numbers
    - 4.3.5 Garbage collection
    - 4.3.6 JSValue
    - 4.3.7 Function call
  - 4.4 RegExp
  - 4.5 Unicode
  - 4.6 BigInt
- 5 License

## 1 Introduction

QuickJS (version 2026-06-04) is a small and embeddable Javascript engine. It supports most of the ES2025 specification 1.

### 1.1 Main Features

- Small and easily embeddable: just a few C files, no external dependency, 210 KiB of x86 code for a simple “hello world” program.
- Fast interpreter with very low startup time: runs the tests of the ECMAScript Test Suite2 in less than 2 minutes on a single core of a desktop PC. The complete life cycle of a runtime instance completes in less than 300 microseconds.
- Almost complete ES2025 support.
- Passes nearly 100% of the ECMAScript Test Suite tests when selecting the ES2025 features.
- Compile Javascript sources to executables with no external dependency.
- Garbage collection using reference counting (to reduce memory usage and have deterministic behavior) with cycle removal.
- Command line interpreter with contextual colorization and completion implemented in Javascript.
- Small built-in standard library with C library wrappers.

## 2 Usage

### 2.1 Installation

A Makefile is provided to compile the engine on Linux or MacOS/X. A preliminary Windows support is available thru cross compilation on a Linux host with the MingGW tools.

Edit the top of the `Makefile` if you wish to select specific options then run `make`.

You can type `make install` as root if you wish to install the binaries and support files to `/usr/local` (this is not necessary to use QuickJS).

Note: On some OSes atomic operations are not available or need a specific library. If you get related errors, you should either add `-latomics` in the Makefile `LIBS` variable or disable `CONFIG_ATOMICS` in `quickjs.c`.

### 2.2 Quick start

`qjs` is the command line interpreter (Read-Eval-Print Loop). You can pass Javascript files and/or expressions as arguments to execute them:

    ./qjs examples/hello.js

`qjsc` is the command line compiler:

    ./qjsc -o hello examples/hello.js
    ./hello

generates a `hello` executable with no external dependency.

### 2.3 Command line options

#### 2.3.1 `qjs` interpreter

    usage: qjs [options] [file [args]]

Options are:

`-h`
`--help`

  List options.

`-e `EXPR``
`--eval `EXPR``

  Evaluate EXPR.

`-i`
`--interactive`

  Go to interactive mode (it is not the default when files are provided on the command line).

`-m`
`--module`

  Load as ES6 module (default=autodetect). A module is autodetected if the filename extension is `.mjs` or if the first keyword of the source is `import`.

`--script`

  Load as ES6 script (default=autodetect).

`-I file`
`--include file`

  Include an additional file.

Advanced options are:

`--std`

  Make the `std` and `os` modules available to the loaded script even if it is not a module.

`-d`
`--dump`

  Dump the memory usage stats.

`-q`
`--quit`

  just instantiate the interpreter and quit.

#### 2.3.2 `qjsc` compiler

    usage: qjsc [options] [files]

Options are:

`-c`

  Only output bytecode in a C file. The default is to output an executable file.

`-e`

  Output `main()` and bytecode in a C file. The default is to output an executable file.

`-o output`

  Set the output filename (default = `out.c` or `a.out`).

`-N cname`

  Set the C name of the generated data.

`-m`

  Compile as Javascript module (default=autodetect).

`-D module_name`

  Compile a dynamically loaded module and its dependencies. This option is needed when your code uses the `import` keyword or the `os.Worker` constructor because the compiler cannot statically find the name of the dynamically loaded modules.

`-M module_name[,cname]`

  Add initialization code for an external C module. See the `c_module` example.

`-x`

  Byte swapped output (only used for cross compilation).

`-flto`

  Use link time optimization. The compilation is slower but the executable is smaller and faster. This option is automatically set when the `-fno-x` options are used.

`-fno-[eval|string-normalize|regexp|json|proxy|map|typedarray|promise|bigint]`

  Disable selected language features to produce a smaller executable file.

### 2.4 Built-in tests

Run `make test` to run the few built-in tests included in the QuickJS archive.

### 2.5 Test262 (ECMAScript Test Suite)

A test262 runner is included in the QuickJS archive. The test262 tests can be installed in the QuickJS source directory with:

    git clone https://github.com/tc39/test262.git test262
    cd test262
    patch -p1 < ../tests/test262.patch
    cd ..

The patch adds the implementation specific `harness` functions and optimizes the inefficient RegExp character classes and Unicode property escapes tests (the tests themselves are not modified, only a slow string initialization function is optimized).

The tests can be run with

    make test2

The configuration files `test262.conf` (resp. `test262o.conf` for the old ES5.1 tests3)) contain the options to run the various tests. Tests can be excluded based on features or filename.

The file `test262_errors.txt` contains the current list of errors. The runner displays a message when a new error appears or when an existing error is corrected or modified. Use the `-u` option to update the current list of errors (or `make test2-update`).

The file `test262_report.txt` contains the logs of all the tests. It is useful to have a clearer analysis of a particular error. In case of crash, the last line corresponds to the failing test.

Use the syntax `./run-test262 -c test262.conf -f filename.js` to run a single test. Use the syntax `./run-test262 -c test262.conf N` to start testing at test number `N`.

For more information, run `./run-test262` to see the command line options of the test262 runner.

`run-test262` accepts the `-N` option to be invoked from `test262-harness`4 thru `eshost`. Unless you want to compare QuickJS with other engines under the same conditions, we do not recommend to run the tests this way as it is much slower (typically half an hour instead of about 100 seconds).

## 3 Specifications

### 3.1 Language support

#### 3.1.1 ES2025 support

The ES2025 specification is almost fully supported including the Annex B (legacy web compatibility) and the Unicode related features.

The following features are not supported yet:

- Tail calls5
- Atomics.waitAsync

#### 3.1.2 ECMA402

ECMA402 (Internationalization API) is not supported.

### 3.2 Modules

ES6 modules are fully supported. The default name resolution is the following:

- Module names with a leading `.` or `..` are relative to the current module path.
- Module names without a leading `.` or `..` are system modules, such as `std` or `os`.
- Module names ending with `.so` are native modules using the QuickJS C API.

### 3.3 Standard library

The standard library is included by default in the command line interpreter. It contains the two modules `std` and `os` and a few global objects.

#### 3.3.1 Global objects

`scriptArgs`

  Provides the command line arguments. The first argument is the script name.

`print(...args)`

  Print the arguments separated by spaces and a trailing newline.

`console.log(...args)`

  Same as print().

#### 3.3.2 `std` module

The `std` module provides wrappers to the libc `stdlib.h` and `stdio.h` and a few other utilities.

Available exports:

`exit(n)`

  Exit the process.

`evalScript(str, options = undefined)`

  Evaluate the string `str` as a script (global eval). `options` is an optional object containing the following optional properties:

  `backtrace_barrier`

    Boolean (default = false). If true, error backtraces do not list the stack frames below the evalScript.

  `async`

    Boolean (default = false). If true, `await` is accepted in the script and a promise is returned. The promise is resolved with an object whose `value` property holds the value returned by the script.

`loadScript(filename)`

  Evaluate the file `filename` as a script (global eval).

`loadFile(filename)`

  Load the file `filename` and return it as a string assuming UTF-8 encoding. Return `null` in case of I/O error.

`open(filename, flags, errorObj = undefined)`

  Open a file (wrapper to the libc `fopen()`). Return the FILE object or `null` in case of I/O error. If `errorObj` is not undefined, set its `errno` property to the error code or to 0 if no error occured.

`popen(command, flags, errorObj = undefined)`

  Open a process by creating a pipe (wrapper to the libc `popen()`). Return the FILE object or `null` in case of I/O error. If `errorObj` is not undefined, set its `errno` property to the error code or to 0 if no error occured.

`fdopen(fd, flags, errorObj = undefined)`

  Open a file from a file handle (wrapper to the libc `fdopen()`). Return the FILE object or `null` in case of I/O error. If `errorObj` is not undefined, set its `errno` property to the error code or to 0 if no error occured.

`tmpfile(errorObj = undefined)`

  Open a temporary file. Return the FILE object or `null` in case of I/O error. If `errorObj` is not undefined, set its `errno` property to the error code or to 0 if no error occured.

`puts(str)`

  Equivalent to `std.out.puts(str)`.

`printf(fmt, ...args)`

  Equivalent to `std.out.printf(fmt, ...args)`.

`sprintf(fmt, ...args)`

  Equivalent to the libc sprintf().

`in`
`out`
`err`

  Wrappers to the libc file `stdin`, `stdout`, `stderr`.

`SEEK_SET`
`SEEK_CUR`
`SEEK_END`

  Constants for seek().

`Error`

  Enumeration object containing the integer value of common errors (additional error codes may be defined):

  `EINVAL`
  `EIO`
  `EACCES`
  `EEXIST`
  `ENOSPC`
  `ENOSYS`
  `EBUSY`
  `ENOENT`
  `EPERM`
  `EPIPE`
`strerror(errno)`

  Return a string that describes the error `errno`.

`gc()`

  Manually invoke the cycle removal algorithm. The cycle removal algorithm is automatically started when needed, so this function is useful in case of specific memory constraints or for testing.

`getenv(name)`

  Return the value of the environment variable `name` or `undefined` if it is not defined.

`setenv(name, value)`

  Set the value of the environment variable `name` to the string `value`.

`unsetenv(name)`

  Delete the environment variable `name`.

`getenviron()`

  Return an object containing the environment variables as key-value pairs.

`urlGet(url, options = undefined)`

  Download `url` using the `curl` command line utility. `options` is an optional object containing the following optional properties:

  `binary`

    Boolean (default = false). If true, the response is an ArrayBuffer instead of a string. When a string is returned, the data is assumed to be UTF-8 encoded.

  `full`

    Boolean (default = false). If true, return the an object contains the properties `response` (response content), `responseHeaders` (headers separated by CRLF), `status` (status code). `response` is `null` is case of protocol or network error. If `full` is false, only the response is returned if the status is between 200 and 299. Otherwise `null` is returned.

`parseExtJSON(str)`

  Parse `str` using a superset of `JSON.parse`. The superset is very close to the JSON5 specification. The following extensions are accepted:

  - Single line and multiline comments
  - unquoted properties (ASCII-only Javascript identifiers)
  - trailing comma in array and object definitions
  - single quoted strings
  - `\v` escape and multi-line strings with trailing `\`
  - `\f` and `\v` are accepted as space characters
  - leading plus or decimal point in numbers
  - hexadecimal (`0x` prefix), octal (`0o` prefix) and binary (`0b` prefix) integers
  - `NaN` and `Infinity` are accepted as numbers

FILE prototype:

`close()`

  Close the file. Return 0 if OK or `-errno` in case of I/O error.

`puts(str)`

  Outputs the string with the UTF-8 encoding.

`printf(fmt, ...args)`

  Formatted printf.

  The same formats as the standard C library `printf` are supported. Integer format types (e.g. `%d`) truncate the Numbers or BigInts to 32 bits. Use the `l` modifier (e.g. `%ld`) to truncate to 64 bits.

`flush()`

  Flush the buffered file.

`seek(offset, whence)`

  Seek to a give file position (whence is `std.SEEK_*`). `offset` can be a number or a bigint. Return 0 if OK or `-errno` in case of I/O error.

`tell()`

  Return the current file position.

`tello()`

  Return the current file position as a bigint.

`eof()`

  Return true if end of file.

`fileno()`

  Return the associated OS handle.

`error()`

  Return true if there was an error.

`clearerr()`

  Clear the error indication.

`read(buffer, position, length)`

  Read `length` bytes from the file to the ArrayBuffer `buffer` at byte position `position` (wrapper to the libc `fread`).

`write(buffer, position, length)`

  Write `length` bytes to the file from the ArrayBuffer `buffer` at byte position `position` (wrapper to the libc `fwrite`).

`getline()`

  Return the next line from the file, assuming UTF-8 encoding, excluding the trailing line feed.

`readAsString(max_size = undefined)`

  Read `max_size` bytes from the file and return them as a string assuming UTF-8 encoding. If `max_size` is not present, the file is read up its end.

`getByte()`

  Return the next byte from the file. Return -1 if the end of file is reached.

`putByte(c)`

  Write one byte to the file.

#### 3.3.3 `os` module

The `os` module provides Operating System specific functions:

- low level file access
- signals
- timers
- asynchronous I/O
- workers (threads)

The OS functions usually return 0 if OK or an OS specific negative error code.

Available exports:

`open(filename, flags, mode = 0o666)`

  Open a file. Return a handle or < 0 if error.

`O_RDONLY`
`O_WRONLY`
`O_RDWR`
`O_APPEND`
`O_CREAT`
`O_EXCL`
`O_TRUNC`

  POSIX open flags.

`O_TEXT`

  (Windows specific). Open the file in text mode. The default is binary mode.

`close(fd)`

  Close the file handle `fd`.

`seek(fd, offset, whence)`

  Seek in the file. Use `std.SEEK_*` for `whence`. `offset` is either a number or a bigint. If `offset` is a bigint, a bigint is returned too.

`read(fd, buffer, offset, length)`

  Read `length` bytes from the file handle `fd` to the ArrayBuffer `buffer` at byte position `offset`. Return the number of read bytes or < 0 if error.

`write(fd, buffer, offset, length)`

  Write `length` bytes to the file handle `fd` from the ArrayBuffer `buffer` at byte position `offset`. Return the number of written bytes or < 0 if error.

`isatty(fd)`

  Return `true` is `fd` is a TTY (terminal) handle.

`ttyGetWinSize(fd)`

  Return the TTY size as `[width, height]` or `null` if not available.

`ttySetRaw(fd)`

  Set the TTY in raw mode.

`remove(filename)`

  Remove a file. Return 0 if OK or `-errno`.

`rename(oldname, newname)`

  Rename a file. Return 0 if OK or `-errno`.

`realpath(path)`

  Return `[str, err]` where `str` is the canonicalized absolute pathname of `path` and `err` the error code.

`getcwd()`

  Return `[str, err]` where `str` is the current working directory and `err` the error code.

`chdir(path)`

  Change the current directory. Return 0 if OK or `-errno`.

`mkdir(path, mode = 0o777)`

  Create a directory at `path`. Return 0 if OK or `-errno`.

`stat(path)`
`lstat(path)`

  Return `[obj, err]` where `obj` is an object containing the file status of `path`. `err` is the error code. The following fields are defined in `obj`: dev, ino, mode, nlink, uid, gid, rdev, size, blocks, atime, mtime, ctime. The times are specified in milliseconds since 1970. `lstat()` is the same as `stat()` excepts that it returns information about the link itself.

`S_IFMT`
`S_IFIFO`
`S_IFCHR`
`S_IFDIR`
`S_IFBLK`
`S_IFREG`
`S_IFSOCK`
`S_IFLNK`
`S_ISGID`
`S_ISUID`

  Constants to interpret the `mode` property returned by `stat()`. They have the same value as in the C system header `sys/stat.h`.

`utimes(path, atime, mtime)`

  Change the access and modification times of the file `path`. The times are specified in milliseconds since 1970. Return 0 if OK or `-errno`.

`symlink(target, linkpath)`

  Create a link at `linkpath` containing the string `target`. Return 0 if OK or `-errno`.

`readlink(path)`

  Return `[str, err]` where `str` is the link target and `err` the error code.

`readdir(path)`

  Return `[array, err]` where `array` is an array of strings containing the filenames of the directory `path`. `err` is the error code.

`setReadHandler(fd, func)`

  Add a read handler to the file handle `fd`. `func` is called each time there is data pending for `fd`. A single read handler per file handle is supported. Use `func = null` to remove the handler.

`setWriteHandler(fd, func)`

  Add a write handler to the file handle `fd`. `func` is called each time data can be written to `fd`. A single write handler per file handle is supported. Use `func = null` to remove the handler.

`signal(signal, func)`

  Call the function `func` when the signal `signal` happens. Only a single handler per signal number is supported. Use `null` to set the default handler or `undefined` to ignore the signal. Signal handlers can only be defined in the main thread.

`SIGINT`
`SIGABRT`
`SIGFPE`
`SIGILL`
`SIGSEGV`
`SIGTERM`

  POSIX signal numbers.

`kill(pid, sig)`

  Send the signal `sig` to the process `pid`.

`exec(args[, options])`

  Execute a process with the arguments `args`. `options` is an object containing optional parameters:

  `block`

    Boolean (default = true). If true, wait until the process is terminated. In this case, `exec` return the exit code if positive or the negated signal number if the process was interrupted by a signal. If false, do not block and return the process id of the child.

  `usePath`

    Boolean (default = true). If true, the file is searched in the `PATH` environment variable.

  `file`

    String (default = `args[0]`). Set the file to be executed.

  `cwd`

    String. If present, set the working directory of the new process.

  `stdin`
  `stdout`
  `stderr`

    If present, set the handle in the child for stdin, stdout or stderr.

  `env`

    Object. If present, set the process environment from the object key-value pairs. Otherwise use the same environment as the current process.

  `uid`

    Integer. If present, the process uid with `setuid`.

  `gid`

    Integer. If present, the process gid with `setgid`.

`getpid()`

  Return the current process ID.

`waitpid(pid, options)`

  `waitpid` Unix system call. Return the array `[ret, status]`. `ret` contains `-errno` in case of error.

`WNOHANG`

  Constant for the `options` argument of `waitpid`.

`dup(fd)`

  `dup` Unix system call.

`dup2(oldfd, newfd)`

  `dup2` Unix system call.

`pipe()`

  `pipe` Unix system call. Return two handles as `[read_fd, write_fd]` or null in case of error.

`sleep(delay_ms)`

  Sleep during `delay_ms` milliseconds.

`sleepAsync(delay_ms)`

  Asynchronouse sleep during `delay_ms` milliseconds. Returns a promise. Example:

      await os.sleepAsync(500);

`now()`

  Return a timestamp in milliseconds with more precision than `Date.now()`. The time origin is unspecified and is normally not impacted by system clock adjustments.

`setTimeout(func, delay)`

  Call the function `func` after `delay` ms. Return a handle to the timer.

`clearTimeout(handle)`

  Cancel a timer.

`platform`

  Return a string representing the platform: `"linux"`, `"darwin"`, `"win32"` or `"js"`.

`Worker(module_filename)`

  Constructor to create a new thread (worker) with an API close to the `WebWorkers`. `module_filename` is a string specifying the module filename which is executed in the newly created thread. As for dynamically imported module, it is relative to the current script or module path. Threads normally don’t share any data and communicate between each other with messages. Nested workers are not supported. An example is available in `tests/test_worker.js`.

  The worker class has the following static properties:

  `parent`

    In the created worker, `Worker.parent` represents the parent worker and is used to send or receive messages.

  The worker instances have the following properties:

  `postMessage(msg)`

    Send a message to the corresponding worker. `msg` is cloned in the destination worker using an algorithm similar to the `HTML` structured clone algorithm. `SharedArrayBuffer` are shared between workers.

    Current limitations: `Map` and `Set` are not supported yet.

  `onmessage`

    Getter and setter. Set a function which is called each time a message is received. The function is called with a single argument. It is an object with a `data` property containing the received message. The thread is not terminated if there is at least one non `null` `onmessage` handler.

### 3.4 QuickJS C API

The C API was designed to be simple and efficient. The C API is defined in the header `quickjs.h`.

#### 3.4.1 Runtime and contexts

`JSRuntime` represents a Javascript runtime corresponding to an object heap. Several runtimes can exist at the same time but they cannot exchange objects. Inside a given runtime, no multi-threading is supported.

`JSContext` represents a Javascript context (or Realm). Each JSContext has its own global objects and system objects. There can be several JSContexts per JSRuntime and they can share objects, similar to frames of the same origin sharing Javascript objects in a web browser.

#### 3.4.2 JSValue

`JSValue` represents a Javascript value which can be a primitive type or an object. Reference counting is used, so it is important to explicitly duplicate (`JS_DupValue()`, increment the reference count) or free (`JS_FreeValue()`, decrement the reference count) JSValues.

#### 3.4.3 C functions

C functions can be created with `JS_NewCFunction()`. `JS_SetPropertyFunctionList()` is a shortcut to easily add functions, setters and getters properties to a given object.

Unlike other embedded Javascript engines, there is no implicit stack, so C functions get their parameters as normal C parameters. As a general rule, C functions take constant `JSValue`s as parameters (so they don’t need to free them) and return a newly allocated (=live) `JSValue`.

#### 3.4.4 Exceptions

Exceptions: most C functions can return a Javascript exception. It must be explicitly tested and handled by the C code. The specific `JSValue` `JS_EXCEPTION` indicates that an exception occurred. The actual exception object is stored in the `JSContext` and can be retrieved with `JS_GetException()`.

#### 3.4.5 Script evaluation

Use `JS_Eval()` to evaluate a script or module source.

If the script or module was compiled to bytecode with `qjsc`, it can be evaluated by calling `js_std_eval_binary()`. The advantage is that no compilation is needed so it is faster and smaller because the compiler can be removed from the executable if no `eval` is required.

Note: the bytecode format is linked to a given QuickJS version. Moreover, no security check is done before its execution. Hence the bytecode should not be loaded from untrusted sources. That’s why there is no option to output the bytecode to a binary file in `qjsc`.

#### 3.4.6 JS Classes

C opaque data can be attached to a Javascript object. The type of the C opaque data is determined with the class ID (`JSClassID`) of the object. Hence the first step is to register a new class ID and JS class (`JS_NewClassID()`, `JS_NewClass()`). Then you can create objects of this class with `JS_NewObjectClass()` and get or set the C opaque point with `JS_GetOpaque()`/`JS_SetOpaque()`.

When defining a new JS class, it is possible to declare a finalizer which is called when the object is destroyed. The finalizer should be used to release C resources. It is invalid to execute JS code from it. A `gc_mark` method can be provided so that the cycle removal algorithm can find the other objects referenced by this object. Other methods are available to define exotic object behaviors.

The Class ID are globally allocated (i.e. for all runtimes). The JSClass are allocated per `JSRuntime`. `JS_SetClassProto()` is used to define a prototype for a given class in a given JSContext. `JS_NewObjectClass()` sets this prototype in the created object.

Examples are available in `quickjs-libc.c`.

#### 3.4.7 C Modules

Native ES6 modules are supported and can be dynamically or statically linked. Look at the `test_bjson` and `bjson.so` examples. The standard library `quickjs-libc.c` is also a good example of a native module.

#### 3.4.8 Memory handling

Use `JS_SetMemoryLimit()` to set a global memory allocation limit to a given JSRuntime.

Custom memory allocation functions can be provided with `JS_NewRuntime2()`.

The maximum system stack size can be set with `JS_SetMaxStackSize()`.

#### 3.4.9 Execution timeout and interrupts

Use `JS_SetInterruptHandler()` to set a callback which is regularly called by the engine when it is executing code. This callback can be used to implement an execution timeout.

It is used by the command line interpreter to implement a `Ctrl-C` handler.

## 4 Internals

### 4.1 Bytecode

The compiler generates bytecode directly with no intermediate representation such as a parse tree, hence it is very fast. Several optimizations passes are done over the generated bytecode.

A stack-based bytecode was chosen because it is simple and generates compact code.

For each function, the maximum stack size is computed at compile time so that no runtime stack overflow tests are needed.

A separate compressed line number table is maintained for the debug information.

Access to closure variables is optimized and is almost as fast as local variables.

Direct `eval` in strict mode is optimized.

### 4.2 Executable generation

#### 4.2.1 `qjsc` compiler

The `qjsc` compiler generates C sources from Javascript files. By default the C sources are compiled with the system compiler (`gcc` or `clang`).

The generated C source contains the bytecode of the compiled functions or modules. If a full complete executable is needed, it also contains a `main()` function with the necessary C code to initialize the Javascript engine and to load and execute the compiled functions and modules.

Javascript code can be mixed with C modules.

In order to have smaller executables, specific Javascript features can be disabled, in particular `eval` or the regular expressions. The code removal relies on the Link Time Optimization of the system compiler.

#### 4.2.2 Binary JSON

`qjsc` works by compiling scripts or modules and then serializing them to a binary format. A subset of this format (without functions or modules) can be used as binary JSON. The example `test_bjson.js` shows how to use it.

Warning: the binary JSON format may change without notice, so it should not be used to store persistent data. The `test_bjson.js` example is only used to test the binary object format functions.

### 4.3 Runtime

#### 4.3.1 Strings

Strings are stored either as an 8 bit or a 16 bit array of characters. Hence random access to characters is always fast.

The C API provides functions to convert Javascript Strings to C UTF-8 encoded strings. The most common case where the Javascript string contains only ASCII characters involves no copying.

#### 4.3.2 Objects

The object shapes (object prototype, property names and flags) are shared between objects to save memory.

Arrays with no holes (except at the end of the array) are optimized.

TypedArray accesses are optimized.

#### 4.3.3 Atoms

Object property names and some strings are stored as Atoms (unique strings) to save memory and allow fast comparison. Atoms are represented as a 32 bit integer. Half of the atom range is reserved for immediate integer literals from 0 to 2^{31}-1.

#### 4.3.4 Numbers

Numbers are represented either as 32-bit signed integers or 64-bit IEEE-754 floating point values. Most operations have fast paths for the 32-bit integer case.

#### 4.3.5 Garbage collection

Reference counting is used to free objects automatically and deterministically. A separate cycle removal pass is done when the allocated memory becomes too large. The cycle removal algorithm only uses the reference counts and the object content, so no explicit garbage collection roots need to be manipulated in the C code.

#### 4.3.6 JSValue

It is a Javascript value which can be a primitive type (such as Number, String, ...) or an Object. NaN boxing is used in the 32-bit version to store 64-bit floating point numbers. The representation is optimized so that 32-bit integers and reference counted values can be efficiently tested.

In 64-bit code, JSValue are 128-bit large and no NaN boxing is used. The rationale is that in 64-bit code memory usage is less critical.

In both cases (32 or 64 bits), JSValue exactly fits two CPU registers, so it can be efficiently returned by C functions.

#### 4.3.7 Function call

The engine is optimized so that function calls are fast. The system stack holds the Javascript parameters and local variables.

### 4.4 RegExp

A specific regular expression engine was developed. It is both small and efficient and supports all the ES2025 features including the Unicode properties. As the Javascript compiler, it directly generates bytecode without a parse tree.

Backtracking with an explicit stack is used so that there is no recursion on the system stack. Simple quantifiers are specifically optimized to avoid recursions.

The full regexp library weights about 15 KiB (x86 code), excluding the Unicode library.

### 4.5 Unicode

A specific Unicode library was developed so that there is no dependency on an external large Unicode library such as ICU. All the Unicode tables are compressed while keeping a reasonable access speed.

The library supports case conversion, Unicode normalization, Unicode script queries, Unicode general category queries and all Unicode binary properties.

The full Unicode library weights about 45 KiB (x86 code).

### 4.6 BigInt

BigInts are represented using binary two’s complement notation. An additional short bigint value is used to optimize the performance on small BigInt values.

## 5 License

QuickJS is released under the MIT license.

Unless otherwise specified, the QuickJS sources are copyright Fabrice Bellard and Charlie Gordon.

#### Footnotes

### (1)

https://tc39.es/ecma262/2025

### (2)

https://github.com/tc39/test262

### (3)

The old ES5.1 tests can be extracted with `git clone --single-branch --branch es5-tests https://github.com/tc39/test262.git test262o`

### (4)

https://github.com/bterlson/test262-harness

### (5)

We believe the current specification of tails calls is too complicated and presents limited practical interests.