Cannot build - "undefined" is not valid JSON [plugin angular-less] - with sourcemaps enabled angular.json

[thaoula]

Command

build

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

19.1.x

Description

Hi Team,

Just tried upgrading to Angular 19.2.0 and have hit an issue when attempting to build our large Angular project.

I keep getting the following error -
"undefined" is not valid JSON [plugin angular-less]

We have 5 different projects in our Angular.json and 3 of them build and 2 present this error. The exact same error repeats 33 times for 1 of the projects and only 2 time for another project.

If i disable sourcemaps in angular.json I can build.

Minimal Reproduction

1. ng update @angular/cli @angular/core
2. ng build

Exception or Error

✘ [ERROR] "undefined" is not valid JSON [plugin angular-less]

    node_modules/@angular/build/src/tools/esbuild/stylesheets/less-language.js:189:21:
      189 │     const map = JSON.parse(sourceMap);
          ╵                      ^

    at JSON.parse (<anonymous>)
    at sourceMapToUrlComment (/Users/thaoula/projects/jms-platform/jms-web/node_modules/@angular/build/src/tools/esbuild/stylesheets/less-language.js:189:22)
    at compileString (/Users/thaoula/projects/jms-platform/jms-web/node_modules/@angular/build/src/tools/esbuild/stylesheets/less-language.js:134:54)
    at async processStylesheet (/Users/thaoula/projects/jms-platform/jms-web/node_modules/@angular/build/src/tools/esbuild/stylesheets/stylesheet-plugin-factory.js:180:18)

  This error came from the "onLoad" callback registered here:

    node_modules/@angular/build/src/tools/esbuild/stylesheets/stylesheet-plugin-factory.js:125:22:
      125 │                 build.onLoad({ filter: language.fileFilter, names...
          ╵                       ~~~~~~

    at setup (/Users/thaoula/projects/jms-platform/jms-web/node_modules/@angular/build/src/tools/esbuild/stylesheets/stylesheet-plugin-factory.js:125:23)
    at handlePlugins (/Users/thaoula/projects/jms-platform/jms-web/node_modules/esbuild/lib/main.js:1151:21)

Your Environment

Angular CLI: 19.2.0
Node: 20.18.1
Package Manager: npm 10.8.2
OS: darwin arm64

Angular: 19.2.0
... animations, cli, common, compiler, compiler-cli, core, forms
... localize, platform-browser, platform-browser-dynamic
... platform-server, router, ssr

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1902.0
@angular-devkit/build-angular   19.2.0
@angular-devkit/core            19.2.0
@angular-devkit/schematics      19.2.0
@angular/cdk                    19.2.1
@angular/cdk-experimental       19.2.1
@schematics/angular             19.2.0
rxjs                            7.8.2
typescript                      5.7.3
zone.js                         0.15.0

Anything else relevant?

No response

[Noxichek]

@thaoula do u use Taiga UI?

[thaoula]

@Noxichek No we do not use Taiga UI.

[behdi]

We have the exact same problem (with a different error message), but disabling the sourcemaps seems to fix it. Issue did not exist in 19.1.x. Would appreciate a fix :)

[clydin]

Would it be possible to create a minimal reproduction that causes this problem? A fix is pending but an additional test case would be useful to prevent regressions.

[thaoula]

Would it be possible to create a minimal reproduction that causes this problem? A fix is pending but an additional test case would be useful to prevent regressions.

@clydin can you give me a hint on where to look. Like i said, we have 5 projects in our Angular project. 3 build and 2 do not and return this error. Also, the of the two that do not build. one of them output's the error 33 times and the other project just 2 times.

[rseiffert]

@thaoula do u use Taiga UI?

I have the same problem, and I am using Taiga UI 4.26.0

[talaat-elesh-rr]

same issue on my side and NO Taiga UI
PrimeNG, Tailwind, Preline UI are the only three main libs in my project!

[clydin]

The underlying issue unfortunately appears to be with less itself. It is being configured to provide sourcemap outputs but under some conditions it appears to not do so. We have a fix to handle this case but ideally understanding what conditions trigger less to not generate a sourcemap when configured to do so would allow an additional test case to be added.

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