module: add SourceMap.lineLengths

Fix: #48460

Author: isaacs

doc/api/module.md

@@ -274,6 +274,7 @@ added:
 #### `new SourceMap(payload)`
 
 * `payload` {Object}
+* `lineLengths` {number\[]}
 
 Creates a new `sourceMap` instance.
 
@@ -287,6 +288,9 @@ Creates a new `sourceMap` instance.
 * `mappings`: {string}
 * `sourceRoot`: {string}
 
+`lineLengths` is an array of the length of each line in the
+generated code.
+
 #### `sourceMap.payload`
 
 * Returns: {Object}

lib/internal/source_map/source_map.js

@@ -71,6 +71,7 @@ const {
   ArrayPrototypePush,
   ArrayPrototypeSlice,
   ArrayPrototypeSort,
+  ArrayLength,
   ObjectPrototypeHasOwnProperty,
   StringPrototypeCharAt,
 } = primordials;
@@ -125,12 +126,13 @@ class SourceMap {
   #mappings = [];
   #sources = {};
   #sourceContentByURL = {};
+  #lineLengths = undefined;
 
   /**
    * @constructor
    * @param {SourceMapV3} payload
    */
-  constructor(payload) {
+  constructor(payload, lineLengths) {
     if (!base64Map) {
       const base64Digits =
              'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
@@ -140,6 +142,9 @@ class SourceMap {
     }
     this.#payload = cloneSourceMapV3(payload);
     this.#parseMappingPayload();
+    if (ArrayIsArray(lineLengths) && ArrayLength(lineLengths)) {
+      this.#lineLengths = lineLengths;
+    }
   }
 
   /**
@@ -149,6 +154,16 @@ class SourceMap {
     return cloneSourceMapV3(this.#payload);
   }
 
+  /**
+   * @return {number[] | undefined} line lengths of generated source code
+   */
+  get lineLengths() {
+    if (this.#lineLengths) {
+      return ArrayPrototypeSlice(this.#lineLengths);
+    }
+    return undefined;
+  }
+
   #parseMappingPayload = () => {
     if (this.#payload.sections) {
       this.#parseSections(this.#payload.sections);

lib/internal/source_map/source_map_cache.js

@@ -317,7 +317,7 @@ function findSourceMap(sourceURL) {
   }
   let sourceMap = entry.sourceMap;
   if (sourceMap === undefined) {
-    sourceMap = new SourceMap(entry.data);
+    sourceMap = new SourceMap(entry.data, entry.lineLengths);
     entry.sourceMap = sourceMap;
   }
   return sourceMap;

test/parallel/test-source-map-api.js

@@ -49,6 +49,8 @@ const { readFileSync } = require('fs');
   assert.strictEqual(originalLine, 2);
   assert.strictEqual(originalColumn, 4);
   assert(originalSource.endsWith('disk.js'));
+  assert(Array.isArray(sourceMap.lineLengths));
+  assert(!sourceMap.lineLengths.some((len) => (typeof len !== 'number')));
 }
 
 // findSourceMap() can be used in Error.prepareStackTrace() to lookup
@@ -96,7 +98,10 @@ const { readFileSync } = require('fs');
   const payload = JSON.parse(readFileSync(
     require.resolve('../fixtures/source-map/disk.map'), 'utf8'
   ));
-  const sourceMap = new SourceMap(payload);
+  const lineLengths = readFileSync(
+    require.resolve('../fixtures/source-map/disk.map'), 'utf8'
+  ).replace(/\n$/, '').split('\n').map((l) => l.length);
+  const sourceMap = new SourceMap(payload, lineLengths);
   const {
     originalLine,
     originalColumn,
@@ -105,6 +110,11 @@ const { readFileSync } = require('fs');
   assert.strictEqual(originalLine, 2);
   assert.strictEqual(originalColumn, 4);
   assert(originalSource.endsWith('disk.js'));
+  const sourceMapLineLengths = sourceMap.lineLengths;
+  for (let i = 0; i < sourceMapLineLengths.length; i++) {
+    assert.strictEqual(sourceMapLineLengths[i], lineLengths[i]);
+  }
+  assert.strictEqual(sourceMapLineLengths.length, lineLengths.length);
   // The stored payload should be a clone:
   assert.strictEqual(payload.mappings, sourceMap.payload.mappings);
   assert.notStrictEqual(payload, sourceMap.payload);