为什么这段代码不会在JavaScript中引发错误？

Question

It took me a while to find this problem in my code when it stopped working.

console.log('before var');
var lastReview = ''
    newReview;
console.log('after var');

There is no comma after lastReview = '' which I believe is a syntax error, however both Chrome inspector and Firebug report nothing in the console. The following code console.log simply is never executed.

Chrome actually reports the error if you run the snippet directly in the console... but not when the actual page runs. Firefox does not report an error in any case.

Why don't the dev tools report these kinds of simple problems?

Answer 1

There is no (syntax) error - this code is valid javascript. According to the rules , JS inserts a semicolon:

console.log('before var');
var lastReview = ''; <-- here
    newReview;
console.log('after var');

so that newReview; becomes a valid, albeit nonsensical, statement.

Although there's no syntax error, this code still throws a runtime error due to undefined variable (assuming newReview wasn't declared before).

Answer 2

Because semi-colons are automatically inserted anyway if missing - so there is no syntax error in the code.

Read more about this here in the Rules of Automatic Semicolon Insertion section of the ECMAScript Language Specification.

In your example, this:

var lastReview = ''

becomes:

var lastReview = '';