Undefined and Null

Since WebGPU is defined first as a JavaScript API, it uses the JavaScript value undefined in many places to indicate the lack of a value.

This is usually used in dictionaries, where, for example, {} and { powerPreference: undefined } are equivalent, and distinct from both { powerPreference: "high-performance" } and { powerPreference: "low-power" }.

It may also be used in functions/methods. For example, GPUBuffer's getMappedRange(optional GPUSize64 offset = 0, optional GPUSize64 size) can be called equivalently as b.getMappedRange(), b.getMappedRange(0), b.getMappedRange(undefined), or b.getMappedRange(undefined, undefined).

To represent undefined in C, webgpu.h uses NULL where possible (anything behind a pointer, including objects), *_UNDEFINED sentinel numeric values (usually UINT32_MAX or similar), *_Undefined enum values (usually 0), or other semantic-specific names (like WGPU_WHOLE_SIZE).

The place that uses the type will define what to do with an undefined or other sentinel value. It may be:

Required, in which case an error is produced.
Defaulting, in which case it trivially defaults to some other value.
Optional, in which case the API accepts the sentinel value and handles it (usually this is either a special value or it has more complex defaulting, for example depending on other values).

C-Specific Sentinel Values

Undefined and null values are also used in C-specific ways in place of WebIDL's more flexible typing:

WGPUStringView has a special null value
WGPUFuture has a special null value
Special cases to indicate the parent struct is null, avoiding extra layers of pointers just for nullability:
Nullable Floating-Point Type