Can identifiers start with a digit in JavaScript?

Which of these is an invalid identifier?

A). foo_bar

B). fooBar

C). foo-bar

D). foo$bar

What will the following code output: var x; x = 5; var y = x;

A). undefined

B). null

C). 5

D). x

Which characters are allowed in JavaScript identifiers?

A). Letters only

B). Letters and numbers only

C). Letters, numbers, $ and _

D). Any special characters

Which of the following is a reserved word in JavaScript?

A). myVariable

B). function

C). let

D). number

What will happen if you declare a variable with the name undefined?

A). Syntax error

B). Declares a variable

C). Throws a runtime error

D). Reassigns undefined

What is the maximum length of an identifier?

A). 255 characters

B). 31 characters

C). 64 characters

D). No limit

Which of the following is an invalid identifier?

A). _variable

B). var$

C). 2variable

D). variable2

Can an identifier be a single character?

A). Yes

B). No

C). Only in ES5

D). Only in ES6

Which is a correct identifier for a variable?

A). 9variable

B). variable-9

C). variable_9

D). variable 9

What is the output of the following code: var x = 5, x = 10;

A). Syntax error

B). 5

C). 10

D). Reference error