Interesting Error Based On Automatic Semicolon Insertion Js Rules. Explanation Required

Today I wrote code for some programming contest. When I run it I was surprised because of error. Cannot read property 'forEach' of undefined in a place where looks like 'error free

Solution 1:

1,2,3 === 3 

comma operator

so

[1,2,3] -> [3]i.e. is property named 3

bracket notation

therefore

sum = 0[3] is undefined - 

since the Number(0) has no property called 3

to illustrate further

var sum = 0
[1,2,3,'constructor']

console.log(sum);

now the code is equivalent to

var sum = 0['constructor']

or

var sum = 0..constructor

which, as you see in the console is the Number object constructor

Solution 2:

The problem is that when you don't put a semi-colon it doesn't automatically put ; instead

• it tries to get the property 3 of a number.
• Here [1,2,3] doesnot work as array but as Bracket Notation.
• Due to Comma Operator the last operand 3.
• So the 0[3] is undefined.undefined.forEach(){...} will obviously throw error.

let sum = 0
[1,2,3]
console.log(sum)