Question

If {}object, e.g. {}"string", {}[1, 2, 3], or {}({}) is exactly equal (according to ===) to object, e.g. "string", [1, 2, 3], or ({}), why can you define a variable with the latter but not the former?

To clarify:

{}"string" === "string" // true
var a = "string"        // No error
var a = {}"string"      // SyntaxError: Unexpected string
var a = ({}"string")    // SyntaxError: Unexpected string
var a = {}("string")    // TypeError: object is not a function
var a = ({}("string"))  // TypeError: object is not a function

Answer 1

In this context, the {} seems to be behaving like an empty block, not as an object literal. So think of it syntactically like:

// Valid syntax...
// despite confusing whitespace...
for (var i=0;i<5;i++) {}"string" === "string";

However, that empty block cannot be used on the right side of an assignment like

var a = {}"string"; //SyntaxError

And here, the {}() implies that {} is being used as a function, with parameters inside ()

var a = {}("string")    // TypeError: object is not a function