it’s definitely a ⁜little⁜ funky because it calls ToString on ⁜the resulting primitive⁜ not on the original value so

"that is " + {
toString () { return "false" },
valueOf () { return true },
}

returns "that is true"

compare

({ false: "good", true: "bad" }[{
toString () { return "false" },
valueOf () { return true },
}])

for the people who aren’t up on the ECMAScript standard, ToPrimitive, ToString, and ToNumeric are abstract operations defined in the standard, not functions callable from within Javascript code

anyway + fails if :—

1. ToPrimitive throws an error, for example because it does not produce a primitive

2. one of the resulting primitive values is a string, and ToString throws an error, because the primitive value cannot be converted to a string (symbols are the only primitive value which can’t be converted to strings by ToString atm)

3. neither primitive value is a string, and ToNumeric throws an error, for example because it does not produce a numeric value

4. The numeric values produced by calling ToNumeric on both sides are different types (presently the available types are numbers and bigints)

this is, to be clear, binary plus

unary plus just calls ToNumber

as far as i am aware, ToPrimitive does not throw an error for any builtin JavaScript object type

but you can make your own objects which throw errors by defining [Symbol.toPrimitive]() on them