Indicators for Impurity

• void functions
• Accessing (global) mutable state
• Date/Time functions (Date, setTimeout, setInterval, ...)
• Clock-based functions (Math.random, UUIDs, ...)
• Exception Handling (try-catch-finally)
• I/O (console.log, file-io, ajax-calls, rest-calls, ...)

Mutating Array Functions

• copyWithin()
• fill()
• sort()
• reverse()
• pop()
• push()
• shift()
• unshift()

All other Array Functions are non-mutating