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.

Node 6.10.3 seems to work fine, just FYI

DrC
– DrC

2017-06-06 01:56:39 +00:00
Commented Jun 6, 2017 at 1:56 — DrC
– DrC, Commented Jun 6, 2017 at 1:56

zzzzBov · Accepted Answer · 2017-06-06 01:57:23Z

21

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']

answered Jun 6, 2017 at 1:57

zzzzBov

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1 Comment

rmacqueen Over a year ago

Very interesting! I didn't know about ASI, and it's propagation effects. Thanks!

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