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A single-line comment in JavaScript is a lexical construct that instructs the JavaScript parser to ignore a specific sequence of characters, omitting them from the execution context during the tokenization phase.

Syntax and Parsing Mechanics

A single-line comment is initiated by the double forward-slash token (//). The JavaScript engine ignores all characters following this token up to the next LineTerminator (such as a line feed \n, carriage return \r, line separator \u2028, or paragraph separator \u2029).

Placement

Single-line comments can be declared as standalone lines or appended to the end of a valid JavaScript statement. When appended inline, the parser evaluates the preceding code and ignores everything from the // token onward.

Lexical Boundaries

Because the single-line comment is strictly terminated by a LineTerminator, it cannot span multiple lines. Any text on a subsequent line is treated as standard executable code by the engine unless it is preceded by another // token.
If a // token is placed inside a string literal or a regular expression, it is evaluated as standard character data rather than a comment initiator.
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