![]() ![]() The most significant bit is set on all but the last character in the VLQ the remaining five bits are prepended to the integer value represented. Each character represents a particular 6-bit value. These deltas tend to be smaller than their absolute values, which means they are more compact when encoded: = Įach character in a VLQ is drawn from a set of 64 printable ASCII characters comprising the upper- and lower-case letters, the decimal digits, and some symbols. a given value is the delta since the previous value. Every value is relative to the last occurrence of its type, i.e. Filenames and associated names are encoded as indices into side tables stored in other fields of the source map’s JSON object. When consecutive mappings are on the same generated JavaScript line, they are separated by a comma: = ' ' Įach individual mapping has a location in the generated JavaScript, and optionally a location in the original source text, which might also contain an associated name: = ] Įvery component of a mapping is a Variable Length Quantity (VLQ) encoded integer. Mappings are grouped by generated JavaScript line number, which is incremented every time a semicolon lays between individual mappings. We will describe the "mappings" string’s grammar in extended Backus-Naur form (EBNF). The "mappings" field is a string that makes up the bulk of the source map, and contains the bidirectional mappings between source and object locations. debug_line section.Ī source map is a JSON object with a handful of fields. Source maps encode debug information similar to that found in DWARF’s. JavaScript developer tools use source maps to symbolicate backtraces, and to implement source-level stepping in debuggers. The source map format provides a bidirectional mapping between locations in some generated JavaScript code that was emitted by a compiler 0, minifier, or package bundling tool back to locations in the original sources that a programmer authored. Subsequent Breakpoints and Pauses at Exceptions.Pausing at an Exception for the First Time.Setting a Breakpoint for the First Time.Parsing and Querying Source Maps in Rust. ![]() We hope that, by sharing our experience, we inspire others to follow suit and rewrite performance-sensitive JavaScript in Rust via WebAssembly. We removed JavaScript code that had been written in a convoluted and unidiomatic way in the name of performance, and replaced it with idiomatic Rust code that performs even better. Additionally, performance is also more consistent: relative standard deviations decreased. ![]() The WebAssembly is up to 5.89 times faster than the JavaScript implementation on realistic benchmarks operating on real world source maps. Tom Tromey and I have replaced the most performance-sensitive portions of the source-map JavaScript Library’s source map parser with Rust code that is compiled to WebAssembly. Read about these extra gains in Speed Without Wizardry! Edit: Further algorithmic improvements yielded additional speedups over what is described here, for total speedups of up to 10.9x faster than the original implementation. ![]()
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