Uncaught Syntaxerror: Unexpected Token Instanceof (with Chrome Javascript Console)

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 instanceo

Solution 1:

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.

({}) instanceofObject

Solution 2:

{}, 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.

({}) instanceofObject;

Solution 3:

If you try this:

var a = {}
a instanceofObject

outputs true, which is the expected output.

However, in your case

{} instanceofObject

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.

Solution 4:

Try

var p = {}
p instanceofObject