bengourley · April 29, 2025 23:45 · Mar 23, 2018
diff --git a/decode.js b/decode.js
@@ -0,0 +1,50 @@
+//
+// This program reads a sourcemap from stdin
+// and replaces the "mappings" property with
+// human readable content. It writes the output
+// to stdout.
+// 
+// 1. install the dependencies:
+//    npm i concat-stream vlq
+//
+// 2. optional: install jq for pretty printing json
+//
+// 3. run the command like so:
+//
+//      cat my-source-map.js.map | node decode | jq .
+//
+
+const concat = require('concat-stream')
+const vlq = require('vlq')
+
+const formatMappings = (mappings, sources, names) => {
+  const vlqState = [ 0, 0, 0, 0, 0 ]
+  return mappings.split(';').reduce((accum, line, i) => {
+    accum[i + 1] = formatLine(line, vlqState, sources, names)
+    vlqState[0] = 0
+    return accum
+  }, {})
+}
+
+const formatLine = (line, state, sources, names) => {
+  const segs = line.split(',')
+  return segs.map(seg => {
+    if (!seg) return ''
+    const decoded = vlq.decode(seg)
+    for (var i = 0; i < 5; i++) {
+      state[i] = typeof decoded[i] === 'number' ? state[i] + decoded[i] : state[i]
+    }
+    return formatSegment(...state.concat([ sources, names ]))
+  })
+}
+
+const formatSegment = (col, source, sourceLine, sourceCol, name, sources, names) =>
+  `${col + 1} => ${sources[source]} ${sourceLine + 1}:${sourceCol + 1}${names[name] ? ` ${names[name]}` : ``}`
+
+process.stdin.pipe(concat((json) => {
+  const map = JSON.parse(json)
+  process.stdout.write(JSON.stringify({
+    ...map,
+    mappings: formatMappings(map.mappings, map.sources, map.names)
+  }))
+}))
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