I am surprised that the following code when input into the Chrome js console:

{} instanceof Object

results in this error message:

Uncaught SyntaxError: Unexpected token instanceof

Can anyone please tell me why that is and how to fix it?

I am surprised that the following code when input into the Chrome js console:

{} instanceof Object

results in this error message:

Uncaught SyntaxError: Unexpected token instanceof

Can anyone please tell me why that is and how to fix it?

• javascript
• google-chrome-devtools

4 Answers 4

The grammar for instanceof is:

RelationalExpression instanceof ShiftExpression

per ECMA-262 §11.8.

The punctuator { at the start of a statement is seen as the start of a block, so the following } closes the block and ends the statement.

The following instanceof operator is the start of the next statement, but it can't be at the start because it must be preceded by a RelationalExpression, so the parser gets a surprise.

You need to force {} to be seen as an object literal by putting something else at the start of the statement, e.g.

({}) instanceof Object

{}, in that context, is a block, not an object literal.

You need change the context (e.g. by wrapping it in ( and )) to make it an object literal.

({}) instanceof Object;

If you try this:

var a = {}
a instanceof Object

outputs true, which is the expected output.

However, in your case

{} instanceof Object

The above doesn't outputs true.

The latter isn't the same as the first one. In the first case, we create an object literal, while in the second case we don't. Hence you get this error.

Try

var p = {}
p instanceof Object