Put parens around the object initializer:
data.map((d) => ({id: d.id, selected: bool}) );
Parentheses have no effect on the value of the expression inside them, but they do have the syntactic effect of eliminating the ambiguity of the first token of the contained expression. Without the parentheses, the JavaScript parser has to decide whether the { token means “Here starts a function body” or “Here starts an object initializer.” It always chooses the former (that is, a block of code).
Introducing the parentheses, therefore, eliminates the confusion: the only thing that a leading ( can mean is “here comes an expression”, so that { inside the parentheses can only be “here comes an object initializer.” (You can’t drop a block of code in the middle of an expression, in other words; if you try, then you’ll get a syntax error.)