Why is 0[0] syntactically valid?

Question

Why is this line valid in javascript ?

var a = 0[0];

After that, a is undefined.

Answer 1

Like most programming languages, JS uses a grammar to parse your code and convert it to an executable form. If there's no rule in the grammar that can be applied to a particular chunk of code, it throws a SyntaxError. Otherwise, the code is considered valid, no matter if it makes sense or not.

The relevant parts of the JS grammar are

Literal :: 
   NumericLiteral
   ...

PrimaryExpression :
   Literal
   ...

MemberExpression :
   PrimaryExpression
   MemberExpression [ Expression ]
   ...

Since 0[0] conforms to these rules, it's considered a valid expression. Whether it's correct (e.g. doesn't throw an error at run time) is another story, but yes it is. This is how JS evaluates expressions like someLiteral[someExpression]:

1. evaluate someExpression (which can be arbitrary complex)
2. convert the literal to a corresponding object type (numeric literals => Number, strings => String etc)
3. call the get property operation on result(2) with the property name result(1)
4. discard result(2)

So 0[0] is interpreted as

index = 0
temp = Number(0)
result = getproperty(temp, index) // it's undefined, but JS doesn't care
delete temp
return result

Here's a example of a valid, but incorrect expression:

null[0]

It's parsed fine, but at run time, the interpreter fails on step 2 (because null can't be converted to an object) and throws a run time error.