How to generate 1 to 1 Source Map? - source-maps

Question

How to generate Source Map if the Source and Target are exactly the same?
One possible way is to write mapping for every single character in the file, like
x in source == x in target
It works, but the resulting Source Map size is huge. Is there any better way to do it?
P.S.
Why do I need such a strange thing? Because I need to join multiple source maps (batch).
a.js -- (possible) transformations --> |
... | -- join --> batch.js.map
x.js -- (possible) transformations --> |
In development we don't use minifications (so there's no initial source map for JS file), but still use batches, and I need source map (1to1 would be fine) to produce source map for the batch (I post-process source maps with special processor that sets correct offsets for batch source map, etc., but it needs a source map to start with).

Answer 1

There are several options to create an identity source map:
You could use the generate-source-map package. It works only for javascript files; it parses them and generates one mapping for each javascript token (operators, identifiers etc). Best quality maps but at the cost of generation time and map size.
var generate = require('generate-source-map');
var fs = require('fs');
var file = 'test.js';
var map = generate({
source: fs.readFileSync(file),
sourceFile: file
});
fs.writeFileSync(file + '.map', map.toString());
Then there's the more generic source-list-map. It works for any text file and generates 1-to-1 line mappings. Less detailed, probably faster and smaller maps.
var SourceListMap = require('source-list-map').SourceListMap;
var fs = require('fs');
var file = 'test.js';
var sourceListMap = new SourceListMap();
var fileContents = fs.readFileSync(file).toString();
sourceListMap.add(fileContents, file, fileContents);
var map = sourceListMap.toStringWithSourceMap({ file: file });
fs.writeFileSync(file + '.map', JSON.stringify(map));
For ultimate flexibility you could use the source-map package and do the splitting yourself:
var sourceMap = require('source-map');
var sourceMapResolve = require('source-map-resolve');
var fs = require('fs');
var file = 'test.js';
var fileContents = fs.readFileSync(file).toString();
var chunks = [];
var line = 1;
fileContents.match(/^[^\r\n]*(?:\r\n?|\n?)/mg).forEach(function(token) {
if (!/^\s*$/.test(token)) {
chunks.push(new sourceMap.SourceNode(line, 0, file, token));
}
++line;
});
var node = new sourceMap.SourceNode(null, null, null, chunks);
node.setSourceContent(file, fileContents);
var result = node.toStringWithSourceMap({file: file});
fs.writeFileSync(file + '.map', result.map);
The two last ones also provide additional functionality e.g. for merging multiple files together.