CODEWITHSUNDEEP
javascript
Juraj Pecháč
Juraj Pecháč

Reputation: 137

Nonexistent semicolon

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?

Upvotes: 2

Views: 86

Answers (2)

Rob Kwasowski
Rob Kwasowski

Reputation: 2780

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)

Upvotes: 0

CertainPerformance
CertainPerformance

Reputation: 370729

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.

Upvotes: 7

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