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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.

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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). 
// The parser ignores all characters on this line
let x = 10;

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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. 
let y = 20; // The parser evaluates 'let y = 20;' and ignores this text

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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. 
// The parser ignores this line
let z = 30; // The parser evaluates the declaration, then ignores the rest of the line
// The parser ignores this line as well
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. 
let path = "https://example.com"; // The first '//' is part of the string literal
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