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A multi-line comment in JavaScript is a lexical construct that instructs the JavaScript engine’s parser to ignore a specific sequence of characters during the tokenization phase. The syntax is defined by a starting delimiter /* and a terminating delimiter */. During lexical analysis, the parser evaluates multi-line comments as whitespace (specifically, as token separators) rather than performing zero-width removal. Because they do not simply disappear to merge adjacent characters, inserting a comment within a single identifier or keyword splits the token and results in a SyntaxError. 
/*
  The parser ignores this sequence.
  It can span multiple lines.
*/
let x = 10;

// Evaluates as: let total = 100 + 50;
let total = 100 + /* inline token separator */ 50;

// Evaluates as: let my Var = 1; (Throws SyntaxError)
let my/*comment*/Var = 1;

​
Automatic Semicolon Insertion (ASI)

According to the ECMAScript specification, if a multi-line comment contains one or more line breaks, the parser treats the entire comment as a LineTerminator rather than standard whitespace. This distinction is critical because a LineTerminator can unexpectedly trigger Automatic Semicolon Insertion (ASI) when following restricted productions such as return, break, continue, throw, or yield. 
function evaluateASI() {
  return /* 
  */ true;
}

// The line break inside the comment acts as a LineTerminator.
// The parser applies ASI, evaluating the code as:
// return;
// true;
// The function returns `undefined` instead of `true`.

​
Technical Limitations: Nesting

JavaScript multi-line comments cannot be nested. The lexical scanner reads the comment sequentially and terminates the comment block at the very first instance of the closing delimiter */. If a multi-line comment contains another multi-line comment, the parser treats the inner comment’s closing delimiter as the end of the outer comment. The parser then resumes standard tokenization for the remaining text, which results in a SyntaxError when it eventually encounters the outer comment’s dangling closing delimiter. 
/*
  Initiating outer comment block.
  
  /* 
    Initiating inner comment block.
  */ 
  
  // The parser resumes execution here. 
  // This is valid JavaScript, so it is successfully tokenized and evaluated.
  let z = 5; 
  
*/ // SyntaxError: Unexpected token '*'
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