Error calling forEach on an array literal in javascript [duplicate]

Question

This code generates an error when I run it with node v6.9.2

var req = {}

['foo', 'bar'].forEach(prop => {
    console.log("prop: " + prop)
});

The error is: TypeError: Cannot read property 'forEach' of undefined

Why would that be? There doesn't seem to be anything syntactically wrong with what I'm doing. I note that if I add a semi-colon after the var req = {} line the error disappears, but I still don't understand why since I thought semi-colons were optional in JavaScript as long as each statement was on a separate line.

Answer 1

Automatic semicolon insertion has turned

var req = {}
['foo', 'bar']

into

var req = {}['foo', 'bar']

and of course {} doesn't contain a 'bar' property (because 'foo', 'bar' evaluates to 'bar').

(undefined).forEach(...) gets evaluated, which then leads to the error that you're seeing.

---

If you're relying on ASI, it's common practice to prefix lines beginning with [] or () with a semicolon:

var req = {}
;['foo', 'bar']