Question

If I type

}{

in the console of Firefox or Chrome, the result is undefined, not a SyntaxError.

This construction return undefined (construction like }!{ return false). What is the reason for such behavior?

Answer 1

When you enter debugger; in the console, a break point shows up, which reveals the answer:

with ((window && window.console && window.console._commandLineAPI) || {}) {
debugger;
}

In Chrome’s developer tools, the input is wrapped inside a with block, then literally evaluated. So, when you put in }{, the following is evaluated:

with ((window && window.console && window.console._commandLineAPI) || {}) {
}{
}

This is an empty with block, followed by an empty block.

Exercise to reader: Try }for(;;){