Struct mozjs::jsapi::JS::Value

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#[repr(C)]
pub struct Value { pub asBits_: u64, }
Expand description

[SMDOC] JS::Value type

JS::Value is the interface for a single JavaScript Engine value. A few general notes on JS::Value:

  • JS::Value has setX() and isX() members for X in

    { Int32, Double, String, Symbol, BigInt, Boolean, Undefined, Null, Object, Magic }

    JS::Value also contains toX() for each of the non-singleton types.

  • Magic is a singleton type whose payload contains either a JSWhyMagic “reason” for the magic value or a uint32_t value. By providing JSWhyMagic values when creating and checking for magic values, it is possible to assert, at runtime, that only magic values with the expected reason flow through a particular value. For example, if cx->exception has a magic value, the reason must be JS_GENERATOR_CLOSING.

  • The JS::Value operations are preferred. The JSVAL_* operations remain for compatibility; they may be removed at some point. These operations mostly provide similar functionality. But there are a few key differences. One is that JS::Value gives null a separate type. Also, to help prevent mistakenly boxing a nullable JSObject* as an object, Value::setObject takes a JSObject&. (Conversely, Value::toObject returns a JSObject&.) A convenience member Value::setObjectOrNull is provided.

  • Note that JS::Value is 8 bytes on 32 and 64-bit architectures. Thus, on 32-bit user code should avoid copying jsval/JS::Value as much as possible, preferring to pass by const Value&.

§Spectre mitigations

To mitigate Spectre attacks, we do the following:

  • On 64-bit platforms, when unboxing a Value, we XOR the bits with the expected type tag (instead of masking the payload bits). This guarantees that toString, toObject, toSymbol will return an invalid pointer (because some high bits will be set) when called on a Value with a different type tag.

  • On 32-bit platforms,when unboxing an object/string/symbol Value, we use a conditional move (not speculated) to zero the payload register if the type doesn’t match.

Fields§

§asBits_: u64

Implementations§

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impl Value

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pub fn is_undefined(&self) -> bool

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pub fn is_null(&self) -> bool

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pub fn is_null_or_undefined(&self) -> bool

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pub fn is_boolean(&self) -> bool

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pub fn is_int32(&self) -> bool

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pub fn is_double(&self) -> bool

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pub fn is_number(&self) -> bool

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pub fn is_primitive(&self) -> bool

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pub fn is_string(&self) -> bool

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pub fn is_object(&self) -> bool

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pub fn is_object_or_null(&self) -> bool

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pub fn is_magic(&self) -> bool

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pub fn is_symbol(&self) -> bool

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pub fn is_bigint(&self) -> bool

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pub fn is_gcthing(&self) -> bool

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pub fn to_boolean(&self) -> bool

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pub fn to_int32(&self) -> i32

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pub fn to_double(&self) -> f64

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pub fn to_number(&self) -> f64

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pub fn to_string(&self) -> *mut JSString

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pub fn to_object(&self) -> *mut JSObject

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pub fn to_object_or_null(&self) -> *mut JSObject

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pub fn to_symbol(&self) -> *mut Symbol

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pub fn to_bigint(&self) -> *mut BigInt

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pub fn to_private(&self) -> *const c_void

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pub fn to_gcthing(&self) -> *mut c_void

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pub fn is_markable(&self) -> bool

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pub fn trace_kind(&self) -> TraceKind

Trait Implementations§

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impl Clone for Value

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fn clone(&self) -> Value

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more
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impl CustomTrace for Value

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impl Debug for Value

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fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
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impl Default for Value

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fn default() -> Value

Returns the “default value” for a type. Read more
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impl GCMethods for Value

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unsafe fn initial() -> Value

Create a default value
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unsafe fn post_barrier(v: *mut Value, prev: Value, next: Value)

Place a post-write barrier
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impl PartialEq for Value

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fn eq(&self, other: &Value) -> bool

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl RootKind for Value

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fn rootKind() -> RootKind

Returns the rooting kind for Self.
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impl Copy for Value

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impl StructuralPartialEq for Value

Auto Trait Implementations§

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impl Freeze for Value

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impl RefUnwindSafe for Value

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impl Send for Value

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impl Sync for Value

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impl Unpin for Value

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impl UnwindSafe for Value

Blanket Implementations§

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T> ToOwned for T
where T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.