progressEvent

ProgressEvent is a standard event object in the Document Object Model (DOM) that represents the progress of a data transfer or resource loading operation. It extends the base Event interface and is commonly emitted by APIs such as XMLHttpRequest, FileReader, and various streaming interfaces during downloads, uploads, or data processing.

The ProgressEvent interface provides three read-only properties. lengthComputable is a boolean indicating whether the total size of the operation is known. loaded is the amount of work completed so far, typically measured in bytes for binary transfers. total is the total size of the operation, also typically in bytes, though the meaning of these units may vary with different APIs. If lengthComputable is false, the total size is not known and progress cannot be expressed as a definite percentage.

Constructors and usage: ProgressEvent objects can be created programmatically with a constructor that takes a type string and an initialization dictionary (ProgressEventInit). In practice, however, these events are usually generated by the browser APIs that report progress. Developers typically handle progress events by checking event.lengthComputable and, when true, computing the percent complete as (event.loaded / event.total) * 100 to update progress indicators.

Implications: ProgressEvent enables user interfaces to reflect ongoing operations, such as showing a progress bar for file uploads or downloads. Its reliability depends on the underlying API and whether the total size is known. It is widely supported in modern browsers and is part of the standard DOM event model.

See also: Event, FileReader, XMLHttpRequest, Fetch, Progress.