Boilerplate

• Property descriptor
• Function length
• Function name
• Is not a constructor
• prototype is Function.prototype
• is extensible

Success cases

• Sum of finite numbers (same sign, mixed signs)
• Sum of one number is that number (negative, -0, 0, positive, NaN, ∞, -∞)
• Sum of empty is -0
• Sum of finite numbers with infinite result (∞, -∞)
• Sum of multiple numbers including any NaN is NaN
• Sum including any ∞ and no NaN or -∞ is ∞
• Sum including any -∞ and no NaN or ∞ is -∞
• Sum of ∞ and -∞ is NaN
• Sum of multiple -0 is -0
• Sum of multiple 0 is 0
• Sum of -0 and 0 is 0
• Sum of a non-array iterable works like sum of an array
• Sum of an empty non-array iterable is -0

Failure cases

• rejects non-Number values in iterable (Boolean, String, Symbol, BigInt, Object, null, undefined)

Other edge cases

• Not passing an argument throws TypeError
• summing each type of primitive throws TypeError
• summing a non-iterable object throws TypeError
• iterable yields more than 2⁵³ items (probably not testable)
• A case where sumPrecise disagrees with naive sum via reduce
• Order of observable operations
• iterator is closed (return is called) after yielding a non-Number
• an Object value in the iterable does not have its valueOf, toString, or [Symbol.toPrimitive] methods called
• iterator is still exhausted after the sum has reached the NaN or ±∞ states
• array is iterated via its [Symbol.iterator] method, not by getting length