"Encountered a syntax error while attempting to reference an attribute on an empty object

> "[object Number]" === Object.prototype.toString.call(1) // #1
< true
> "[object Number]" === {}.toString.call(1)               // #2
< true
> {}.toString.call(1) === "[object Number]"               // #3
< SyntaxError: Unexpected token '.'
> ({}).toString.call(1) === "[object Number]"             // #4
< true
> {}.toString.call(1)                                     // #5
< SyntaxError: Unexpected token '.'
> !{}.toString.call(1)                                    // #6
< false
> test = {}.toString.call(1)                              // #7
< "[object Number]"

From the above examples, it is evident that #2 and #3 are very similar, but with the positions swapped. While #2 executes without any issues, #3 throws a syntax error. To rectify this issue in #3, using parentheses solves the problem. As observed in cases #5-7, when `{}` is not at the leftmost position, the code works correctly.

So why does this happen?

javascriptsyntaxsyntax-error

Answer №1

When the code encounters rvalue and === operator in scenario #1, JavaScript interprets that the lvalue could be either a value or an expression.

In situation #2, similar to #1, so JavaScript identifies {} as an object literal.

Regarding #3, since JavaScript reads statements from right to left, the {} is seen simply as a curly brace and not an object literal. This leads to issue in #3 as the toString function will not be accessible on curly braces.

For #4, enclosing {} with () grouping operator trick JavaScript into seeing it as an expression. That's why #4 works because {} is evaluated as an object literal and toString becomes available.

As for #5, akin to #3, JavaScript views {} only as a brace since there are no expressions associated with it.

When it comes to #6, similar to #4, where the presence of the expression ! leads JavaScript to evaluate it as an object.

And finally, in #7, mirroring #4 again, due to the assignment operator, JavaScript considers it an expression.

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