Values, expressions, and errors

Programs are built from expressions — pieces of code that produce a value. 2 + 2 is an expression. So is "Kode" + "Trail". Understanding "this code becomes a value" is a quiet superpower.

Expressions evaluate

JavaScript works from the inside out: (2000 + 500) becomes 2500, then glues onto the string. Notice + does double duty — addition for numbers, concatenation for strings. That leads to JavaScript's most famous quirk:

"2" + 2 gives "22" — when one side is a string, JavaScript converts the other side and concatenates. Keep quotes for text and bare digits for math, and this will never bite you.

Errors are guides

Run this broken line:

ReferenceError: greeting is not defined — JavaScript's version of "I've never heard of this name." The error type tells you the category, the message names the culprit. Compare:

Error	Usually means
ReferenceError: x is not defined	typo, or using a name before creating it
SyntaxError: unexpected token	JavaScript couldn't parse it — often a missing ) or quote
TypeError: x is not a function	calling something that isn't callable — often a typo like console.lg