# otto

[![GoDoc Reference](https://pkg.go.dev/badge/github.com/robertkrimen/otto.svg)](https://pkg.go.dev/github.com/robertkrimen/otto)

## Basic Usage

Package otto is a JavaScript parser and interpreter written natively in Go.

To use import it with the following:

```go
import (
   "github.com/robertkrimen/otto"
)
```

Run something in the VM

```go
vm := otto.New()
vm.Run(`
    abc = 2 + 2;
    console.log("The value of abc is " + abc); // 4
`)
```

Get a value out of the VM

```go
if value, err := vm.Get("abc"); err == nil {
    if value_int, err := value.ToInteger(); err == nil {
        fmt.Printf("", value_int, err)
    }
}
```

Set a number

```go
vm.Set("def", 11)
vm.Run(`
    console.log("The value of def is " + def);
    // The value of def is 11
`)
```

Set a string

```go
vm.Set("xyzzy", "Nothing happens.")
vm.Run(`
    console.log(xyzzy.length); // 16
`)
```

Get the value of an expression

```go
value, _ = vm.Run("xyzzy.length")
{
    // value is an int64 with a value of 16
    value, _ := value.ToInteger()
}
```

An error happens

```go
value, err = vm.Run("abcdefghijlmnopqrstuvwxyz.length")
if err != nil {
    // err = ReferenceError: abcdefghijlmnopqrstuvwxyz is not defined
    // If there is an error, then value.IsUndefined() is true
    ...
}
```

Set a Go function

```go
vm.Set("sayHello", func(call otto.FunctionCall) otto.Value {
    fmt.Printf("Hello, %s.\n", call.Argument(0).String())
    return otto.Value{}
})
```

Set a Go function that returns something useful

```go
vm.Set("twoPlus", func(call otto.FunctionCall) otto.Value {
    right, _ := call.Argument(0).ToInteger()
    result, _ := vm.ToValue(2 + right)
    return result
})
```

Use the functions in JavaScript

```go
result, _ = vm.Run(`
    sayHello("Xyzzy");      // Hello, Xyzzy.
    sayHello();             // Hello, undefined

    result = twoPlus(2.0); // 4
`)
```

## Parser

A separate parser is available in the parser package if you're just interested
in building an AST.

[![GoDoc Reference](https://pkg.go.dev/badge/github.com/robertkrimen/otto/parser.svg)](https://pkg.go.dev/github.com/robertkrimen/otto/parser)

Parse and return an AST

```go
filename := "" // A filename is optional
src := `
    // Sample xyzzy example
    (function(){
        if (3.14159 > 0) {
            console.log("Hello, World.");
            return;
        }

        var xyzzy = NaN;
        console.log("Nothing happens.");
        return xyzzy;
    })();
`

// Parse some JavaScript, yielding a *ast.Program and/or an ErrorList
program, err := parser.ParseFile(nil, filename, src, 0)
```

## Setup

You can run (Go) JavaScript from the command line with
[otto](http://github.com/robertkrimen/otto/tree/master/otto).

```shell
go install github.com/robertkrimen/otto/otto@latest
```

Run JavaScript by entering some source on stdin or by giving otto a filename:

```shell
otto example.js
```

## Underscore

Optionally include the JavaScript utility-belt library, underscore, with this
import:

```go
import (
    "github.com/robertkrimen/otto"
    _ "github.com/robertkrimen/otto/underscore"
)

// Now every otto runtime will come loaded with underscore
```

For more information: [underscore](http://github.com/robertkrimen/otto/tree/master/underscore)

## Caveat Emptor

The following are some limitations with otto:

* `use strict` will parse, but does nothing.
* The regular expression engine ([re2/regexp](https://pkg.go.dev/regexp)) is not fully compatible with the ECMA5 specification.
* Otto targets ES5. Some ES6 features e.g. Typed Arrays are not supported, PR's to add functionality are always welcome.

### Regular Expression Incompatibility

Go translates JavaScript-style regular expressions into something that is
"regexp" compatible via `parser.TransformRegExp`. Unfortunately, RegExp requires
backtracking for some patterns, and backtracking is not supported by Go
[re2](https://github.com/google/re2/wiki/syntax).

Therefore, the following syntax is incompatible:

```plaintext
(?=)  // Lookahead (positive), currently a parsing error
(?!)  // Lookahead (backhead), currently a parsing error
\1    // Backreference (\1, \2, \3, ...), currently a parsing error
```

A brief discussion of these limitations: [Regexp (?!re)](https://groups.google.com/forum/?fromgroups=#%21topic/golang-nuts/7qgSDWPIh_E)

More information [about re2](https://github.com/google/re2)

In addition to the above, re2 (Go) has a different definition for `\s`: `[\t\n\f\r
]`. The JavaScript definition, on the other hand, also includes `\v`, Unicode
"Separator, Space", etc.

### Halting Problem

If you want to stop long running executions (like third-party code), you can use
the interrupt channel to do this:

```go
package main

import (
    "errors"
    "fmt"
    "os"
    "time"

    "github.com/robertkrimen/otto"
)

var halt = errors.New("Stahp")

func main() {
    runUnsafe(`var abc = [];`)
    runUnsafe(`
    while (true) {
        // Loop forever
    }`)
}

func runUnsafe(unsafe string) {
    start := time.Now()
    defer func() {
        duration := time.Since(start)
        if caught := recover(); caught != nil {
            if caught == halt {
                fmt.Fprintf(os.Stderr, "Some code took to long! Stopping after: %v\n", duration)
                return
            }
            panic(caught) // Something else happened, repanic!
        }
        fmt.Fprintf(os.Stderr, "Ran code successfully: %v\n", duration)
    }()

    vm := otto.New()
    vm.Interrupt = make(chan func(), 1) // The buffer prevents blocking
    watchdogCleanup := make(chan struct{})
    defer close(watchdogCleanup)

    go func() {
        select {
        case <-time.After(2 * time.Second): // Stop after two seconds
            vm.Interrupt <- func() {
                panic(halt)
            }
        case <-watchdogCleanup:
        }
        close(vm.Interrupt)
    }()

    vm.Run(unsafe) // Here be dragons (risky code)
}
```

Where is `setTimeout` / `setInterval`?

These timing functions are not actually part of the [ECMA-262 specification](https://ecma-international.org/publications-and-standards/standards/ecma-262/).
Typically, they belong to the `window` object (in the browser). It would not be
difficult to provide something like these via Go, but you probably want to wrap
otto in an event loop in that case.

For an example of how this could be done in Go with otto, see [natto](http://github.com/robertkrimen/natto).

Here is some more discussion of the issue:

* [What is Node.js?](http://book.mixu.net/node/ch2.html)
* [Reentrancy (computing)](http://en.wikipedia.org/wiki/Reentrancy_%28computing%29)
* [Perl Safe Signals](https://metacpan.org/pod/Perl::Unsafe::Signals)

## Usage

```go
var ErrVersion = errors.New("version mismatch")
```

### type Error

```go
type Error struct {}
```

An Error represents a runtime error, e.g. a `TypeError`, a `ReferenceError`, etc.

### func (Error) Error

```go
func (err Error) Error() string
```

Error returns a description of the error

```plaintext
    TypeError: 'def' is not a function
```

### func (Error) String

```go
func (err Error) String() string
```

String returns a description of the error and a trace of where the error
occurred.

```plaintext
    TypeError: 'def' is not a function
        at xyz (<anonymous>:3:9)
        at <anonymous>:7:1/
```

### type FunctionCall

```go
type FunctionCall struct {
    This         Value
    ArgumentList []Value
    Otto         *Otto
}
```

FunctionCall is an encapsulation of a JavaScript function call.

### func (FunctionCall) Argument

```go
func (self FunctionCall) Argument(index int) Value
```

Argument will return the value of the argument at the given index.

If no such argument exists, undefined is returned.

### type Object

```go
type Object struct {}
```

Object is the representation of a JavaScript object.

### func (Object) Call

```go
func (self Object) Call(name string, argumentList ...interface{}) (Value, error)
```

Call a method on the object.

It is essentially equivalent to:

```go
var method, _ := object.Get(name)
method.Call(object, argumentList...)
```

An undefined value and an error will result if:

1. There is an error during conversion of the argument list
2. The property is not actually a function
3. An (uncaught) exception is thrown

### func (Object) Class

```go
func (self Object) Class() string
```

Class will return the class string of the object.

The return value will (generally) be one of:

```plaintext
    Object
    Function
    Array
    String
    Number
    Boolean
    Date
    RegExp
```

### func (Object) Get

```go
func (self Object) Get(name string) (Value, error)
```

Get the value of the property with the given name.

### func (Object) Keys

```go
func (self Object) Keys() []string
```

Get the keys for the object

Equivalent to calling Object.keys on the object

### func (Object) Set

```go
func (self Object) Set(name string, value interface{}) error
```

Set the property of the given name to the given value.

An error will result if the setting the property triggers an exception (i.e.
read-only), or there is an error during conversion of the given value.

### func (Object) Value

```go
func (self Object) Value() Value
```

Value will return self as a value.

### type Otto

```go
type Otto struct {
    // Interrupt is a channel for interrupting the runtime. You can use this to halt a long running execution, for example.
    // See "Halting Problem" for more information.
    Interrupt chan func()
}
```

Otto is the representation of the JavaScript runtime. Each instance of Otto has
a self-contained namespace.

### func New

```go
func New() *Otto
```

New will allocate a new JavaScript runtime

### func Run

```go
func Run(src interface{}) (*Otto, Value, error)
```

Run will allocate a new JavaScript runtime, run the given source on the
allocated runtime, and return the runtime, resulting value, and error (if any).

src may be a string, a byte slice, a bytes.Buffer, or an io.Reader, but it MUST
always be in UTF-8.

src may also be a Script.

src may also be a Program, but if the AST has been modified, then runtime
behavior is undefined.

### func (Otto) Call

```go
func (self Otto) Call(source string, this interface{}, argumentList ...interface{}) (Value, error)
```

Call the given JavaScript with a given this and arguments.

If this is nil, then some special handling takes place to determine the proper
this value, falling back to a "standard" invocation if necessary (where this is
undefined).

If source begins with "new " (A lowercase new followed by a space), then Call
will invoke the function constructor rather than performing a function call. In
this case, the this argument has no effect.

```go
// value is a String object
value, _ := vm.Call("Object", nil, "Hello, World.")

// Likewise...
value, _ := vm.Call("new Object", nil, "Hello, World.")

// This will perform a concat on the given array and return the result
// value is [ 1, 2, 3, undefined, 4, 5, 6, 7, "abc" ]
value, _ := vm.Call(`[ 1, 2, 3, undefined, 4 ].concat`, nil, 5, 6, 7, "abc")
```

### func (*Otto) Compile

```go
func (self *Otto) Compile(filename string, src interface{}) (*Script, error)
```

Compile will parse the given source and return a Script value or nil and an
error if there was a problem during compilation.

```go
script, err := vm.Compile("", `var abc; if (!abc) abc = 0; abc += 2; abc;`)
vm.Run(script)
```

### func (*Otto) Copy

```go
func (in *Otto) Copy() *Otto
```

Copy will create a copy/clone of the runtime.

Copy is useful for saving some time when creating many similar runtimes.

This method works by walking the original runtime and cloning each object,
scope, stash, etc. into a new runtime.

Be on the lookout for memory leaks or inadvertent sharing of resources.

### func (Otto) Get

```go
func (self Otto) Get(name string) (Value, error)
```

Get the value of the top-level binding of the given name.

If there is an error (like the binding does not exist), then the value will be
undefined.

### func (Otto) Object

```go
func (self Otto) Object(source string) (*Object, error)
```

Object will run the given source and return the result as an object.

For example, accessing an existing object:

```go
object, _ := vm.Object(`Number`)
```

Or, creating a new object:

```go
object, _ := vm.Object(`({ xyzzy: "Nothing happens." })`)
```

Or, creating and assigning an object:

```go
object, _ := vm.Object(`xyzzy = {}`)
object.Set("volume", 11)
```

If there is an error (like the source does not result in an object), then nil
and an error is returned.

### func (Otto) Run

```go
func (self Otto) Run(src interface{}) (Value, error)
```

Run will run the given source (parsing it first if necessary), returning the
resulting value and error (if any)

src may be a string, a byte slice, a bytes.Buffer, or an io.Reader, but it MUST
always be in UTF-8.

If the runtime is unable to parse source, then this function will return
undefined and the parse error (nothing will be evaluated in this case).

src may also be a Script.

src may also be a Program, but if the AST has been modified, then runtime
behavior is undefined.

### func (Otto) Set

```go
func (self Otto) Set(name string, value interface{}) error
```

Set the top-level binding of the given name to the given value.

Set will automatically apply ToValue to the given value in order to convert it
to a JavaScript value (type Value).

If there is an error (like the binding is read-only, or the ToValue conversion
fails), then an error is returned.

If the top-level binding does not exist, it will be created.

### func (Otto) ToValue

```go
func (self Otto) ToValue(value interface{}) (Value, error)
```

ToValue will convert an interface{} value to a value digestible by
otto/JavaScript.

### type Script

```go
type Script struct {}
```

Script is a handle for some (reusable) JavaScript. Passing a Script value to a
run method will evaluate the JavaScript.

### func (*Script) String

```go
func (self *Script) String() string
```

### type Value

```go
type Value struct {}
```

Value is the representation of a JavaScript value.

### func FalseValue

```go
func FalseValue() Value
```

FalseValue will return a value representing false.

It is equivalent to:

```go
ToValue(false)
```

### func  NaNValue

```go
func NaNValue() Value
```

NaNValue will return a value representing NaN.

It is equivalent to:

```go
ToValue(math.NaN())
```

### func  NullValue

```go
func NullValue() Value
```

NullValue will return a Value representing null.

### func  ToValue

```go
func ToValue(value interface{}) (Value, error)
```

ToValue will convert an interface{} value to a value digestible by
otto/JavaScript

This function will not work for advanced types (struct, map, slice/array, etc.)
and you should use Otto.ToValue instead.

### func  TrueValue

```go
func TrueValue() Value
```

TrueValue will return a value representing true.

It is equivalent to:

```go
ToValue(true)
```

### func UndefinedValue

```go
func UndefinedValue() Value
```

UndefinedValue will return a Value representing undefined.

### func (Value) Call

```go
func (value Value) Call(this Value, argumentList ...interface{}) (Value, error)
```

Call the value as a function with the given this value and argument list and
return the result of invocation. It is essentially equivalent to:

```js
    value.apply(thisValue, argumentList)
```

An undefined value and an error will result if:

1. There is an error during conversion of the argument list
2. The value is not actually a function
3. An (uncaught) exception is thrown

### func (Value) Class

```go
func (value Value) Class() string
```

Class will return the class string of the value or the empty string if value is
not an object.

The return value will (generally) be one of:

```plaintext
    Object
    Function
    Array
    String
    Number
    Boolean
    Date
    RegExp
```

### func (Value) Export

```go
func (self Value) Export() (interface{}, error)
```

Export will attempt to convert the value to a Go representation and return it
via an interface{} kind.

Export returns an error, but it will always be nil. It is present for backwards
compatibility.

If a reasonable conversion is not possible, then the original value is returned.

```plaintext
    undefined   -> nil (FIXME?: Should be Value{})
    null        -> nil
    boolean     -> bool
    number      -> A number type (int, float32, uint64, ...)
    string      -> string
    Array       -> []interface{}
    Object      -> map[string]interface{}
```

### func (Value) IsBoolean

```go
func (value Value) IsBoolean() bool
```

IsBoolean will return true if value is a boolean (primitive).

### func (Value) IsDefined

```go
func (value Value) IsDefined() bool
```

IsDefined will return false if the value is undefined, and true otherwise.

### func (Value) IsFunction

```go
func (value Value) IsFunction() bool
```

IsFunction will return true if value is a function.

### func (Value) IsNaN

```go
func (value Value) IsNaN() bool
```

IsNaN will return true if value is NaN (or would convert to NaN).

### func (Value) IsNull

```go
func (value Value) IsNull() bool
```

IsNull will return true if the value is null, and false otherwise.

### func (Value) IsNumber

```go
func (value Value) IsNumber() bool
```

IsNumber will return true if value is a number (primitive).

### func (Value) IsObject

```go
func (value Value) IsObject() bool
```

IsObject will return true if value is an object.

### func (Value) IsPrimitive

```go
func (value Value) IsPrimitive() bool
```

IsPrimitive will return true if value is a primitive (any kind of primitive).

### func (Value) IsString

```go
func (value Value) IsString() bool
```

IsString will return true if value is a string (primitive).

### func (Value) IsUndefined

```go
func (value Value) IsUndefined() bool
```

IsUndefined will return true if the value is undefined, and false otherwise.

### func (Value) Object

```go
func (value Value) Object() *Object
```

Object will return the object of the value, or nil if value is not an object.

This method will not do any implicit conversion. For example, calling this
method on a string primitive value will not return a String object.

### func (Value) String

```go
func (value Value) String() string
```

String will return the value as a string.

This method will make return the empty string if there is an error.

### func (Value) ToBoolean

```go
func (value Value) ToBoolean() (bool, error)
```

ToBoolean will convert the value to a boolean (bool).

```plaintext
    ToValue(0).ToBoolean() => false
    ToValue("").ToBoolean() => false
    ToValue(true).ToBoolean() => true
    ToValue(1).ToBoolean() => true
    ToValue("Nothing happens").ToBoolean() => true
```

If there is an error during the conversion process (like an uncaught exception),
then the result will be false and an error.

### func (Value) ToFloat

```go
func (value Value) ToFloat() (float64, error)
```

ToFloat will convert the value to a number (float64).

```plaintext
    ToValue(0).ToFloat() => 0.
    ToValue(1.1).ToFloat() => 1.1
    ToValue("11").ToFloat() => 11.
```

If there is an error during the conversion process (like an uncaught exception),
then the result will be 0 and an error.

### func (Value) ToInteger

```go
func (value Value) ToInteger() (int64, error)
```

ToInteger will convert the value to a number (int64).

```plaintext
    ToValue(0).ToInteger() => 0
    ToValue(1.1).ToInteger() => 1
    ToValue("11").ToInteger() => 11
```

If there is an error during the conversion process (like an uncaught exception),
then the result will be 0 and an error.

### func (Value) ToString

```go
func (value Value) ToString() (string, error)
```

ToString will convert the value to a string (string).

```plaintext
    ToValue(0).ToString() => "0"
    ToValue(false).ToString() => "false"
    ToValue(1.1).ToString() => "1.1"
    ToValue("11").ToString() => "11"
    ToValue('Nothing happens.').ToString() => "Nothing happens."
```

If there is an error during the conversion process (like an uncaught exception),
then the result will be the empty string ("") and an error.
