{}.tostring() Uncaught Syntaxerror: Unexpected Token

when I try it in chrome dev tools, it shows Uncaught SyntaxError: Unexpected token . However if when it assign to a: var a={}.toString(); a //[object Object] what caused the dif

Solution 1:

what caused the difference?

The state the parser is in. By default, the parser is in a state where it expects a statement. So in your example in the console, the { looks like the opening of a block to it, not the beginning of an object initializer. (You can also give it an expression at that point, because JavaScript has the concept of the ExpressionStatement, which is a statement consisting entirely of an expression.)

But in your var a={}.toString(); code, the {}.toString() appears on the right-hand side of an assigment, where the parser is expecting an expression, not a statement. So the { starts an object initializer.

If you do something to make the parser expect an expression instead, it'll work in the console too:

({}).toString(); // "[object Object]"

or

+{}.toString(); // NaN, because the `+` tries to turn `[object Object]` into a number and fails

Solution 2:

When you aren't in expression context (as is triggered by being on the right hand side of an assignment, for instance) then {} is a block statement and not an object literal.

Solution 3:

{} is captured first and interpreted as a block, which has no .toString method to call.

If you wrap the object literal in parens, like ({}).toString(), then it works as expected.

This is because of the parsing rule priorities and the {} tokens being overloaded to be both object and block delimiters.

Solution 4:

The parser is interpreting the {} as a code block. You can make it parse correctly by surrounding the braces with parentheses:

({}).toString();