Don't access localStorage if it is not available

[Windvis]

Description

The changes introduced by https://github.com/plausible/plausible-tracker/pull/17 throw exceptions when localStorage is not available. Some browsers completely remove the localStorage key when certain settings are enabled (for example: "disable all cookies"). This adds an extra check to ensure localStorage is available before accessing it.

Related Issue

Fixes https://github.com/plausible/plausible-tracker/issues/25

Screenshots or GIFs (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Merging #31 (879e8bc) into master (0369c0e) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           76        78    +2     
  Branches        19        22    +3     
=========================================
+ Hits            76        78    +2     

Impacted Files	Coverage Δ
src/lib/request.ts	100.00% <100.00%> (ø)

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[ukutaht]

This is now released under 0.3.6