Nonexistent semicolon

Question

alert("There will be an error")
[1, 2].forEach(alert)

Now if I run the code, only the first alert is shown and then we have an error! I know why we have an error (no Automatic semicolon insertion) but I don`t understand error message: Uncaught TypeError: Cannot read property '2' of undefined. How JavaScript interpreter read this code?

I know, my question is : How JavaScript interpreter read this code? Why can JS read property '2' of undefined ? — Juraj Pecháč, Jul 11 '19 at 11:28
[What are the rules for JavaScript's automatic semicolon insertion (ASI)?](https://stackoverflow.com/questions/2846283) — adiga, Jul 11 '19 at 11:28
It sees your code as `alert("There will be an error")[1, 2].forEach(alert)`, i.e. it's all one statement. The first alert returns undefined, so you have `undefined[1,2].forEach(alert)`. So you're trying to access the `2` property of *undefined*. — RobG, Jul 11 '19 at 11:32
Possible duplicate of [What are the rules for JavaScript's automatic semicolon insertion (ASI)?](https://stackoverflow.com/questions/2846283/what-are-the-rules-for-javascripts-automatic-semicolon-insertion-asi) — VLAZ, Jul 11 '19 at 13:29

score 7 · Answer 1 · answered Jul 11 '19 at 11:27

When you have <expression>[...], the interpreter will attempt to look up a property on expression. When the inside of the brackets contains commas, you're invoking the comma operator, which evaluates to the value of the last item in the list. So

foo[1, 2]

is equivalent to

foo[2]

That's exactly what's happening here:

alert("There will be an error")
[1, 2].forEach(alert)

equivalent to

alert("There will be an error")
[2].forEach(alert)

equivalent to

alert("There will be an error")[2].forEach(alert)

equivalent to (without the alert message)

undefined[2].forEach(alert)

That's where the "2" comes from. alert returns undefined, so the error message is Uncaught TypeError: Cannot read property '2' of undefined.

The [1, 2] does not get evaluated as an array, even though it kind of looks like one.

Rob Kwasowski · Answer 2 · 2019-07-11T11:46:26.153

0

If you write JS without semicolons at the end of each line it will sometimes complain if you start a line with:

[, (, or any of the arithmetic operators +, -, *, /.

Explained further here: https://medium.com/@goatslacker/no-you-dont-need-semicolons-148d936b9cf2

So you can avoid this by defining an array as a variable.

alert("There will be an error")
var a = [1, 2]
a.forEach(alert)

edited Jul 11 '19 at 11:46

answered Jul 11 '19 at 11:34

Rob Kwasowski

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"*It will complain*" only if the resulting expression causes a syntax or runtime error. It's fairly common to split long statements over multiple lines, often at a dot property accessor (though I'm not a fan of that). – RobG Jul 11 '19 at 11:45

Nonexistent semicolon

2 Answers2