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JS syntactic quirks

To make a labyrinth, it takes Some good intentions, some mistakes. —A. E. Stallings, “Daedal”

JavaScript is rather hard to parse. Here is an in-depth accounting of its syntactic quirks, with an eye toward actually implementing a parser from scratch.

With apologies to the generous people who work on the standard. Thanks for doing that—better you than me.

Thanks to @bakkot and @mathiasbynens for pointing out several additional quirks.

Problems are rated in terms of difficulty, from (*) = easy to (***) = hard. We’ll start with the easiest problems.

Dangling else (*)

If you know what this is, you may be excused.

Statements like if (EXPR) STMT if (EXPR) STMT else STMT are straight-up ambiguous in the JS formal grammar. The ambiguity is resolved with a line of specification text:

Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.

I love this sentence. Something about it cracks me up, I dunno...

In a recursive descent parser, just doing the dumbest possible thing correctly implements this rule.

A parser generator has to decide what to do. In Yacc, you can use operator precedence for this.

Yacc aside: This should seem a little outrageous at first, as else is hardly an operator. It helps if you understand what Yacc is doing. In LR parsers, this kind of ambiguity in the grammar manifests as a shift-reduce conflict. In this case, when we’ve already parsed if ( Expression ) Statement if ( Expression ) Statement and are looking at else, it’s unclear to Yacc whether to reduce the if-statement or shift else. Yacc does not offer a feature that lets us just say "always shift else here"; but there is a Yacc feature that lets us resolve shift-reduce conflicts in a rather odd, indirect way: operator precedence. We can resolve this conflict by making else higher-precedence than the preceding symbol ).

Alternatively, I believe it’s equivalent to add "[lookahead ≠ else]" at the end of the IfStatement production that doesn’t have an else.

Other ambiguities and informal parts of the spec (*)

Not all of the spec is as formal as it seems at first. Most of the stuff in this section is easy to deal with, but #4 is special.

  1. The lexical grammar is ambiguous: when looking at the characters <<=, there is the question of whether to parse that as one token <<=, two tokens (< <= or << =), or three (< < =).

    Of course every programming language has this, and the fix is one sentence of prose in the spec:

    The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

    This is easy enough for hand-coded lexers, and for systems that are designed to use separate lexical and syntactic grammars. (Other parser generators may need help to avoid parsing functionf(){} as a function.)

  2. The above line of prose does not apply within input elements, in components of the lexical grammar. In those cases, the same basic idea ("maximum munch") is specified using lookahead restrictions at the end of productions:

    LineTerminatorSequence ::
        <LF>
        <CR>[lookahead ≠ <LF>]
        <LS>
        <PS>
        <CR><LF>

    The lookahead restriction prevents a CR LF sequence from being parsed as two adjacent LineTerminatorSequences.

    This technique is used in several places, particularly in NotEscapeSequences.

  3. Annex B.1.4 extends the syntax for regular expressions, making the grammar ambiguous. Again, a line of prose explains how to cope:

    These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.

  4. Annex B.1.2 extends the syntax of string literals to allow legacy octal escape sequences, like \033. It says:

    The syntax and semantics of 11.8.4 is extended as follows except that this extension is not allowed for strict mode code:

    ...followed by a new definition of EscapeSequence.

    So there are two sets of productions for EscapeSequence, and an implementation is required to implement both and dynamically switch between them.

    This means that function f() { "\033"; "use strict"; } is a SyntaxError, even though the octal escape is scanned before we know we're in strict mode.

For another ambiguity, see "Slashes" below.

Unicode quirks

JavaScript source is Unicode and usually follows Unicode rules for thing like identifiers and whitespace, but it has a few special cases: $, _, U+200C ZERO WIDTH NON-JOINER, and U+200D ZERO WIDTH JOINER are legal in identifiers (the latter two only after the first character), and U+FEFF ZERO WIDTH NO-BREAK SPACE (also known as the byte-order mark) is treated as whitespace.

It also allows any code point, including surrogate halves, even though the Unicode standard says that unpaired surrogate halves should be treated as encoding errors.

Legacy octal literals and escape sequences (*)

This is more funny than difficult.

In a browser, in non-strict code, every sequence of decimal digits (not followed by an identifier character) is a NumericLiteral token.

If it starts with 0, with more digits after, then it's a legacy Annex B.1.1 literal. If the token contains an 8 or a 9, it's a decimal number. Otherwise, hilariously, it's octal.

js> [067, 068, 069, 070]
[55, 68, 69, 56]

There are also legacy octal escape sequences in strings, and these have their own quirks. '\07' === '\u{7}', but '\08' !== '\u{8}' since 8 is not an octal digit. Instead '\08' === '\0' + '8', because \0 followed by 8 or 9 is a legacy octal escape sequence representing the null character. (Not to be confused with \0 in strict code, not followed by a digit, which still represents the null character, but doesn't count as octal.)

None of this is hard to implement, but figuring out what the spec says is hard.

Strict mode (*)

(entangled with: lazy compilation)

A script or function can start with this:

"use strict";

This enables "strict mode". Additionally, all classes and modules are strict mode code.

Strict mode has both parse-time and run-time effects. Parse-time effects include:

  • Strict mode affects the lexical grammar: octal integer literals are SyntaxErrors, octal character escapes are SyntaxErrors, and a handful of words like private and interface are reserved (and thus usually SyntaxErrors) in strict mode.

    Like the situation with slashes, this means it is not possible to implement a complete lexer for JS without also parsing—at least enough to detect class boundaries, "use strict" directives in functions, and function boundaries.

  • It’s a SyntaxError to have bindings named eval or arguments in strict mode code, or to assign to eval or arguments.

  • It’s a SyntaxError to have two argument bindings with the same name in a strict function.

    Interestingly, you don’t always know if you’re in strict mode or not when parsing arguments.

    function foo(a, a) {
        "use strict";
    }

    When the implementation reaches the Use Strict Directive, it must either know that foo has two arguments both named a, or switch to strict mode, go back, and reparse the function from the beginning.

    Fortunately an Early Error rule prohibits mixing "use strict" with more complex parameter lists, like function foo(x = eval('')) {.

  • The expression syntax “delete Identifier” and the abominable WithStatement are banned in strict mode.

Conditional keywords (**)

In some programming languages, you could write a lexer that has rules like

  • When you see if, return Token::If.

  • When you see something like apple or arrow or target, return Token::Identifier.

Not so in JavaScript. The input if matches both the terminal if and the nonterminal IdentifierName, both of which appear in the high-level grammar. The same goes for target.

This poses a deceptively difficult problem for table-driven parsers. Such parsers run on a stream of token-ids, but the question of which token-id to use for a word like if or target is ambiguous. The current parser state can't fully resolve the ambiguity: there are cases like class C { get where the token get might match either as a keyword (the start of a getter) or as an IdentifierName (a method or property named get) in different grammatical productions.

All keywords are conditional, but some are more conditional than others. The rules are inconsistent to a tragicomic extent. Keywords like if that date back to JavaScript 1.0 are always keywords except when used as property names or method names. They can't be variable names. Two conditional keywords (await and yield) are in the Keyword list; the rest are not. New syntax that happened to be introduced around the same time as strict mode was awarded keyword status in strict mode. The rules are scattered through the spec. All this interacts with \u0065 Unicode escape sequences somehow. It’s just unbelievably confusing.

(After writing this section, I proposed revisions to the specification to make it a little less confusing.)

  • Thirty-six words are always reserved:

    break case catch class const continue debugger default delete do else enum export extends false finally for function if import in instanceof new null return super switch this throw true try typeof var void while with

    These tokens can't be used as names of variables or arguments. They're always considered special except when used as property names, method names, or import/export names in modules.

    // property names
    let obj = {if: 3, function: 4};
    assert(obj.if == 3);
    
    // method names
    class C {
      if() {}
      function() {}
    }
    
    // imports and exports
    import {if as my_if} from "modulename";
    export {my_if as if};
  • Two more words, yield and await, are in the Keyword list but do not always act like keywords in practice.

    • yield is a Keyword; but it can be used as an identifier, except in generators and strict mode code.

      This means that yield - 1 is valid both inside and outside generators, with different meanings. Outside a generator, it’s subtraction. Inside, it yields the value -1.

      That reminds me of the Groucho Marx line: Outside of a dog, a book is a man’s best friend. Inside of a dog it’s too dark to read.

    • await is like that, but in async functions. Also it’s not a valid identifier in modules.

    Conditional keywords are entangled with slashes: yield /a/g is two tokens in a generator but five tokens elsewhere.

  • In strict mode code, implements, interface, package, private, protected, and public are reserved (via Early Errors rules).

    This is reflected in the message and location information for certain syntax errors:

    SyntaxError: implements is a reserved identifier:
    class implements {}
    ......^
    
    SyntaxError: implements is a reserved identifier:
    function implements() { "use strict"; }
    ....................................^
    
  • let is not a Keyword or ReservedWord. Usually it can be an identifier. It is special at the beginning of a statement or after for ( or for await (.

    var let = [new Date];   // ok: let as identifier
    let v = let;            // ok: let as keyword, then identifier
    let let;           // SyntaxError: banned by special early error rule
    let.length;        // ok: `let .` -> ExpressionStatement
    let[0].getYear();  // SyntaxError: `let [` -> LexicalDeclaration

    In strict mode code, let is reserved.

  • static is similar. It’s a valid identifier, except in strict mode. It’s only special at the beginning of a ClassElement.

    In strict mode code, static is reserved.

  • async is similar, but trickier. It’s an identifier. It is special only if it’s marking an async function, method, or arrow function (the tough case, since you won’t know it’s an arrow function until you see the =>, possibly much later).

    function async() {}   // normal function named "async"
    
    async();        // ok, `async` is an Identifier; function call
    async() => {};  // ok, `async` is not an Identifier; async arrow function
  • of is special only in one specific place in for-of loop syntax.

    var of = [1, 2, 3];
    for (of of of) console.log(of);  // logs 1, 2, 3

    Amazingly, both of the following are valid JS code:

    for (async of => {};;) {}
    for (async of []) {}

    In the first line, async is a keyword and of is an identifier; in the second line it's the other way round.

    Even a simplified JS grammar can't be LR(1) as long as it includes the features used here!

  • get and set are special only in a class or an object literal, and then only if followed by a PropertyName:

    var obj1 = {get: f};      // `get` is an identifier
    var obj2 = {get x() {}};  // `get` means getter
    
    class C1 { get = 3; }     // `get` is an identifier
    class C2 { get x() {} }   // `get` means getter
  • target is special only in new.target.

  • arguments and eval can't be binding names, and can't be assigned to, in strict mode code.

To complicate matters, there are a few grammatical contexts where both IdentifierName and Identifier match. For example, after var { there are two possibilities:

// Longhand properties: BindingProperty -> PropertyName -> IdentifierName
var { xy: v } = obj;    // ok
var { if: v } = obj;    // ok, `if` is an IdentifierName

// Shorthand properties: BindingProperty -> SingleNameBinding -> BindingIdentifier -> Identifier
var { xy } = obj;       // ok
var { if } = obj;       // SyntaxError: `if` is not an Identifier

Escape sequences in keywords

(entangled with: conditional keywords, ASI)

You can use escape sequences to write variable and property names, but not keywords (including contextual keywords in contexts where they act as keywords).

So if (foo) {} and { i\u0066: 0 } are legal but i\u0066 (foo) is not.

And you don't necessarily know if you're lexing a contextual keyword until the next token: ({ g\u0065t: 0 }) is legal, but ({ g\u0065t x(){} }) is not.

And for let it's even worse: l\u0065t by itself is a legal way to reference a variable named let, which means that

let
x

declares a variable named x, while, thanks to ASI,

l\u0065t
x

is a reference to a variable named let followed by a reference to a variable named x.

Early errors (**)

(entangled with: lazy parsing, conditional keywords, ASI)

Some early errors are basically syntactic. Others are not.

This is entangled with lazy compilation: "early errors" often involve a retrospective look at an arbitrarily large glob of code we just parsed, but in Beast Mode we’re not building an AST. In fact we would like to be doing as little bookkeeping as possible.

Even setting that aside, every early error is a special case, and it’s just a ton of rules that all have to be implemented by hand.

Here are some examples of Early Error rules—setting aside restrictions that are covered adequately elsewhere:

  • Rules about names:

    • Rules that affect the set of keywords (character sequences that match IdentifierName but are not allowed as binding names) based on whether or not we’re in strict mode code, or in a Module. Affected identifiers include arguments, eval, yield, await, let, implements, interface, package, private, protected, public, static.

      • One of these is a strangely worded rule which prohibits using yield as a BindingIdentifier. At first blush, this seems like it could be enforced in the grammar, but that approach would make this a valid program, due to ASI:

        let
        yield 0;

        Enforcing the same rule using an Early Error prohibits ASI here. It works by exploiting the detailed inner workings of ASI case 1, and arranging for 0 to be "the offending token" rather than yield.

    • Lexical variable names have to be unique within a scope:

      • Lexical variables (let and const) can’t be declared more than once in a block, or both lexically declared and declared with var.

      • Lexically declared variables in a function body can’t have the same name as argument bindings.

    • A lexical variable can’t be named let.

    • Common-sense rules dealing with unicode escape sequences in identifiers.

  • Common-sense rules about regular expression literals. (They have to actually be valid regular expressions, and unrecognized flags are errors.)

  • The number of string parts that a template string can have is limited to 232 − 1.

  • Invalid Unicode escape sequences, like \7 or \09 or \u{3bjq, are banned in non-tagged templates (in tagged templates, they are allowed).

  • The SuperCall syntax is allowed only in derived class constructors.

  • const x; without an initializer is a Syntax Error.

  • A direct substatement of an if statement, loop statement, or with statement can’t be a labelled function.

  • Early errors are used to hook up cover grammars.

    • Early errors are also used in one case to avoid having to specify a very large refinement grammar when ObjectLiteral almost covers ObjectAssignmentPattern: sorry, too complicated to explain.
  • Early errors are sometimes used to prevent parsers from needing to backtrack too much.

    • When parsing async ( x = await/a/g ), you don't know until the next token if this is an async arrow or a call to a function named async. This means you can't even tokenize properly, because in the former case the thing following x = is two divisions and in the latter case it's an AwaitExpression of a regular expression. So an Early Error forbids having await in parameters at all, allowing parsers to immediately throw an error if they find themselves in this case.

Many strict mode rules are enforced using Early Errors, but others affect runtime semantics.

Boolean parameters (**)

Some nonterminals are parameterized. (Search for “parameterized production” in this spec section.)

Implemented naively (e.g. by macro expansion) in a parser generator, each parameter could nearly double the size of the parser. Instead, the parameters must be tracked at run time somehow.

Lookahead restrictions (**)

(entangled with: restricted productions)

TODO (I implemented this by hacking the entire LR algorithm. Most every part of it is touched, although in ways that seem almost obvious once you understand LR inside and out.)

(Note: It may seem like all of the lookahead restrictions in the spec are really just a way of saying “this production takes precedence over that one”—for example, that the lookahead restriction on ExpressionStatement just means that other productions for statements and declarations take precedence over it. But that isn't accurate; you can't have an ExpressionStatement that starts with {, even if it doesn't parse as a Block or any other kind of statement.)

Automatic Semicolon Insertion (**)

(entangled with: restricted productions, slashes)

Most semicolons at the end of JS statements and declarations “may be omitted from the source text in certain situations”. This is called Automatic Semicolon Insertion, or ASI for short.

The specification for this feature is both very-high-level and weirdly procedural (“When, as the source text is parsed from left to right, a token is encountered...”, as if the specification is telling a story about a browser. As far as I know, this is the only place in the spec where anything is assumed or implied about the internal implementation details of parsing.) But it would be hard to specify ASI any other way.

Wrinkles:

  1. Whitespace is significant (including whitespace inside comments). Most semicolons in the grammar are optional only at the end of a line (or before }, or at the end of the program).

  2. The ending semicolon of a do-while statement is extra optional. You can always omit it.

  3. A few semicolons are never optional, like the semicolons in for (;;).

    This means there’s a semicolon in the grammar that is optionally optional! This one:

    LexicalDeclaration : LetOrConst BindingList ;

    It’s usually optional, but not if this is the LexicalDeclaration in for (let i = 0; i < 9; i++)!

  4. Semicolons are not inserted only as a last resort to avoid SyntaxErrors. That turned out to be too error-prone, so there are also restricted productions (see below), where semicolons are more aggressively inferred.

  5. In implementations, ASI interacts with the ambiguity of slashes (see below).

A recursive descent parser implements ASI by calling a special method every time it needs to parse a semicolon that might be optional. The special method has to peek at the next token and consume it only if it’s a semicolon. This would not be so bad if it weren’t for slashes.

In a parser generator, ASI can be implemented using an error recovery mechanism.

I think the error recovery mechanism in yacc/Bison is too imprecise—when an error happens, it discards states from the stack searching for a matching error-handling rule. The manual says “Error recovery strategies are necessarily guesses.”

But here’s a slightly more careful error recovery mechanism that could do the job:

  1. For each production in the ES spec grammar where ASI could happen, e.g.

    ImportDeclaration ::= `import` ModuleSpecifier `;`
                                                    { import_declaration($2); }
    

    add an ASI production, like this:

    ImportDeclaration ::= `import` ModuleSpecifier [ERROR]
                                       { check_asi(); import_declaration($2); }
    

    What does this mean? This production can be matched, like any other production, but it's a fallback. All other productions take precedence.

  2. While generating the parser, treat [ERROR] as a terminal symbol. It can be included in start sets and follow sets, lookahead, and so forth.

  3. At run time, when an error happens, synthesize an [ERROR] token. Let that bounce through the state machine. It will cause zero or more reductions. Then, it might actually match a production that contains [ERROR], like the ASI production above.

    Otherwise, we’ll get another error—the entry in the parser table for an [ERROR] token at this state will be an error entry. Then we really have a syntax error.

This solves most of the ASI issues:

  • Whitespace sensitivity: That's what check_asi() is for. It should signal an error if we're not at the end of a line.

  • Special treatment of do-while loops: Make an error production, but don't check_asi().

  • Rule banning ASI in EmptyStatement or for(;;): Easy, don't create error productions for those.

    • Banning ASI in for (let x=1 \n x<9; x++): Manually adjust the grammar, copying LexicalDeclaration so that there's a LexicalDeclarationNoASI production used only by for statements. Not a big deal, as it turns out.
  • Slashes: Perhaps have check_asi reset the lexer to rescan the next token, if it starts with /.

  • Restricted productions: Not solved. Read on.

Restricted productions (**)

(entangled with: ASI, slashes)

Line breaks aren’t allowed in certain places. For example, the following is not a valid program:

throw                 // SyntaxError
  new Error();

For another example, this function contains two statements, not one:

function f(g) {
  return              // ASI
    g();
}

The indentation is misleading; actually ASI inserts a semicolon at the end of the first line: return; g();. (This function always returns undefined. The second statement is never reached.)

These restrictions apply even to multiline comments, so the function

function f(g) {
  return /*
    */ g();
}

contains two statements, just as the previous example did.

I’m not sure why these rules exist, but it’s probably because (back in the Netscape days) users complained about the bizarre behavior of automatic semicolon insertion, and so some special do-what-I-mean hacks were put in.

This is specified with a weird special thing in the grammar:

ReturnStatement : return [no LineTerminator here] Expression ;

This is called a restricted production, and it’s unfortunately necessary to go through them one by one, because there are several kinds. Note that the particular hack required to parse them in a recursive descent parser is a little bit different each time.

  • After continue, break, or return, a line break triggers ASI.

    The relevant productions are all statements, and in each case there’s an alternative production that ends immediately with a semicolon: continue ; break ; and return ;.

    Note that the alternative production is not restricted: e.g. a LineTerminator can appear between return and ;:

    if (x)
      return        // ok
        ;
    else
      f();
  • After throw, a line break is a SyntaxError.

  • After yield, a line break terminates the YieldExpression.

    Here the alternative production is simply yield, not yield ;.

  • In a post-increment or post-decrement expression, there can’t be a line break before ++ or --.

    The purpose of this rule is subtle. It triggers ASI and thus prevents syntax errors:

    var x = y       // ok: semicolon inserted here
    ++z;

    Without the restricted production, var x = y ++ would parse successfully, and the “offending token” would be z. It would be too late for ASI.

    However, the restriction can of course also cause a SyntaxError:

    var x = (y
             ++);   // SyntaxError

As we said, recursive descent parsers can implement these rules with hax.

In a generated parser, there are a few possible ways to implement them. Here are three. If you are not interested in ridiculous approaches, you can skip the first two.

  • Treat every token as actually a different token when it appears after a line break: TokenType::LeftParen and TokenType::LeftParenAfterLineBreak. Of course the parser generator can treat these exactly the same in normal cases, and automatically generate identical table entries (or whatever) except in states where there’s a relevant restricted production.

  • Add a special LineTerminator token. Normally, the lexer skips newlines and never emits this token. However, if the current state has a relevant restricted production, the lexer knows this and emits a LineTerminator for the first line break it sees; and the parser uses that token to trigger an error or transition to another state, as appropriate.

  • When in a state that has a relevant restricted production, change states if there’s a line break before the next token. That is, split each such state into two: the one we stay in when there’s not a line break, and the one we jump to if there is a line break.

In all cases it’ll be hard to have confidence that the resulting parser generator is really sound. (That is, it might not properly reject all ambiguous grammars.) I don’t know exactly what property of the few special uses in the ES grammar makes them seem benign.

Slashes (**)

(entangled with: ASI, restricted productions)

When you see / in a JS program, you don’t know if that’s a division operator or the start of a regular expression unless you’ve been paying attention up to that point.

The spec:

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar.

You might think the lexer could treat / as an operator only if the previous token is one that can be the last token of an expression (a set that includes literals, identifiers, this, ), ], and }). To see that this does not work, consider:

{} /x/                  // `/` after `}` is regexp
({} / 2)                // `/` after `}` is division

for (g of /(a)(b)/) {}  // `/` after `of` is regexp
var of = 6; of / 2      // `/` after `of` is division

throw /x/;              // `/` after `throw` is regexp
Math.throw / 2;         // `/` after `throw` is division

++/x/.lastIndex;        // `/` after `++` is regexp
n++ / 2;                // `/` after `++` is division

So how can the spec be implemented?

In a recursive descent parser, you have to tell the lexer which goal symbol to use every time you ask for a token. And you have to make sure, if you look ahead at a token, but don’t consume it, and fall back on another path that can accept a RegularExpressionLiteral or DivPunctuator, that you did not initially lex it incorrectly. We have assertions for this and it is a bit of a nightmare when we have to touch it (which is thankfully rare). Part of the problem is that the token you’re peeking ahead at might not be part of the same production at all. Thanks to ASI, it might be the start of the next statement, which will be parsed in a faraway part of the Parser.

A table-driven parser has it easy here! The lexer can consult the state table and see which kind of token can be accepted in the current state. This is closer to what the spec actually says.

Two minor things to watch out for:

  • The nonterminal ClassTail is used both at the end of ClassExpression, which may be followed by /; and at the end of ClassDeclaration, which may be followed by a RegularExpressionLiteral at the start of the next statement. Canonical LR creates separate states for these two uses of ClassTail, but the LALR algorithm will unify them, creating some states that have both / and RegularExpressionLiteral in the follow set. In these states, determining which terminal is actually allowed requires looking not only at the current state, but at the current stack of states (to see one level of grammatical context).

  • Since this decision depends on the parser state, and automatic semicolon insertion adjusts the parser state, a parser may need to re-scan a token after ASI.

In other kinds of generated parsers, at least the lexical goal symbol can be determined automatically.

Lazy compilation and scoping (**)

(entangled with: arrow functions)

JS engines lazily compile function bodies. During parsing, when the engine sees a function, it switches to a high-speed parsing mode (which I will call “Beast Mode”) that just skims the function and checks for syntax errors. Beast Mode does not compile the code. Beast Mode doesn’t even create AST nodes. All that will be done later, on demand, the first time the function is called.

The point is to get through parsing fast, so that the script can start running. In browsers, <script> compilation usually must finish before we can show the user any meaningful content. Any part of it that can be deferred must be deferred. (Bonus: many functions are never called, so the work is not just deferred but avoided altogether.)

Later, when a function is called, we can still defer compilation of nested functions inside it.

So what? Seems easy enough, right? Well...

Local variables in JS are optimized. To generate reasonable code for the script or function we are compiling, we need to know which of its local variables are used by nested functions, which we are not currently compiling. That is, we must know which variables occur free in each nested function. So scoping, which otherwise could be done as a separate phase of compilation, must be done during parsing in Beast Mode.

Getting the scoping of arrow functions right while parsing is tricky, because it’s often not possible to know when you’re entering a scope until later. Consider parsing (a. This could be the beginning of an arrow function, or not; we might not know until after we reach the matching ), which could be a long way away.

Annex B.3.3 adds extremely complex rules for scoping for function declarations which makes this especially difficult. In

function f() {
    let x = 1;
    return function g() {
        {
            function x(){}
        }
        {
            let x;
        }
        return x;
    };
}

the function g does not use the initial let x, but in

function f() {
    let x = 1;
    return function g() {
        {
            {
                function x(){}
            }
            let x;
        }
        return x;
    };
}

it does.

Here is a good writeup of what's going on in these examples.

Arrow functions, assignment, destructuring, and cover grammars (***)

(entangled with: lazy compilation)

Suppose the parser is scanning text from start to end, when it sees a statement that starts with something like this:

let f = (a

The parser doesn’t know yet if (a ... is ArrowParameters or just a ParenthesizedExpression. Either is possible. We’ll know when either (1) we see something that rules out the other case:

let f = (a + b           // can't be ArrowParameters

let f = (a, b, ...args   // can't be ParenthesizedExpression

or (2) we reach the matching ) and see if the next token is =>.

Probably (1) is a pain to implement.

To keep the language nominally LR(1), the standard specifies a “cover grammar”, a nonterminal named CoverParenthesizedExpressionAndArrowParameterList that is a superset of both ArrowParameters and ParenthesizedExpression. The spec grammar uses the Cover nonterminal in both places, so technically let f = (a + b) => 7; and let f = (a, b, ...args); are both syntactically valid Scripts, according to the formal grammar. But there are Early Error rules (that is, plain English text, not part of the formal grammar) that say arrow functions’ parameters must match ArrowParameters, and parenthesized expressions must match ParenthesizedExpression.

So after the initial parse, the implementation must somehow check that the CoverParenthesizedExpressionAndArrowParameterList really is valid in context. This complicates lazy compilation because in Beast Mode we are not even building an AST. It’s not easy to go back and check.

Something similar happens in a few other cases: the spec is written as though syntactic rules are applied after the fact:

  • CoverCallExpressionAndAsyncArrowHead covers the syntax async(x) which looks like a function call and also looks like the beginning of an async arrow function.

  • ObjectLiteral is a cover grammar; it covers both actual object literals and the syntax propertyName = expr, which is not valid in object literals but allowed in destructuring assignment:

    var obj = {x = 1};  // SyntaxError, by an early error rule
    
    ({x = 1} = {});   // ok, assigns 1 to x
  • LeftHandSideExpression is used to the left of = in assignment, and as the operand of postfix ++ and --. But this is way too lax; most expressions shouldn’t be assigned to:

    1 = 0;  // matches the formal grammar, SyntaxError by an early error rule
    
    null++;   // same
    
    ["a", "b", "c"]++;  // same

Conclusion

What have we learned today?

  • Do not write a JS parser.

  • JavaScript has some syntactic horrors in it. But hey, you don't make the world's most widely used programming language by avoiding all mistakes. You do it by shipping a serviceable tool, in the right circumstances, for the right users.