Github action not caching node_modules across jobs

Question

I have a Github action on pull_request, and am attempting to cache node_modules between jobs. I can only see cache working on a job re-run, but not when opening new PRs, even when there have been no changes to my node_modules.

Is this by design?

Each time it runs in a new PR, I see this output in the job:

Run actions/cache@v2
  with:
    path: ~/.npm
    key: Linux-build-cache-node-modules-83fc6365b85dd061416fccd5993d48c2003c388bb2184fd57f28d1041d9d261e
    restore-keys: Linux-build-cache-node-modules-
  Linux-build-
  Linux-
  
  env:
    cache-name: cache-node-modules
Cache not found for input keys: Linux-build-cache-node-modules-83fc6365b85dd061416fccd5993d48c2003c388bb2184fd57f28d1041d9d261e, Linux-build-cache-node-modules-, Linux-build-, Linux-

Yet, re-running the job will enjoy the cache hit:

Run actions/cache@v2
Received 54525952 of 109978126 (49.6%), 51.8 MBs/sec
Received 109978126 of 109978126 (100.0%), 67.8 MBs/sec
Cache Size: ~105 MB (109978126 B)
/usr/bin/tar --use-compress-program zstd -d -xf /home/runner/work/_temp/0bdf436d-9d4d-4397-b8ed-5910ff503949/cache.tzst -P -C /home/runner/work/argutopia/argutopia
Cache restored successfully
Cache restored from key: Linux-build-cache-node-modules-83fc6365b85dd061416fccd5993d48c2003c388bb2184fd57f28d1041d9d261e

Here is my action yml for reference:

name: PR CI
on:
  pull_request:
    branches: [main, develop]
jobs:
  build:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        node-version: [14.x]
    steps:
      - uses: actions/checkout@v2
        with:
          fetch-depth: 0

      - name: Use Node.js ${{ matrix.node-version }}
        uses: actions/setup-node@v1
        with:
          node-version: ${{ matrix.node-version }}

      - name: Cache node modules
        uses: actions/cache@v2
        id: cache
        env:
          cache-name: cache-node-modules
        with:
          path: ~/.npm
          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('**/package-lock.json') }}
          restore-keys: |
            ${{ runner.os }}-build-${{ env.cache-name }}-
            ${{ runner.os }}-build-
            ${{ runner.os }}-

      - name: Install Dependencies
        if: steps.cache.outputs.cache-hit != 'true'
        run: |
          echo '::debug::deps cache miss - installing deps'
          npm install

Answer 1

Caching ~/.npm wont allow you to skip running npm install. It will save the global npm cache on this machine so that running npm install doesn't incur as much download time, but you'll then still need to build the local node_modules for your project.

If you want to be able to skip your install, you can cache the project node_modules folder as well and skip install on cache hits there, though this could present some issues down the road.