tfw u forget how exactly the + operator works in Javascript
it just calls ToPrimitive on both sides and then :—
• if the result on either side is a string, calls ToString on both sides and concatenates them
• otherwise, it calls ToNumeric on both sides and attempts to add them
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
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
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)