javascript - Cannot read property 'forEach' of undefined

Question

var funcs = []
[1, 2].forEach( (i) => funcs.push( () => i  ) )

Why does it produce the error below?

TypeError: Cannot read property 'forEach' of undefined
    at Object.<anonymous>

However, the error goes away if the semicolon ; is added to the end of the first line.

Answer 1

There is no semicolon at the end of the first line. So the two lines run together, and it is interpreted as setting the value of funcs to

[][1, 2].forEach( (i) => funcs.push( () => i  ) )

The expression 1, 2 becomes just 2 (comma operator), so you're trying to access index 2 of an empty array:

[][2] // undefined

And undefined has no forEach method. To fix this, always make sure you put a semicolon at the end of your lines (or if you don't, make sure you know what you're doing).