SpiderMonkey Parser API: A Standard For Structured JS Representations

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SpiderMonkey Parser API: A Standard For Structured JS Representations Michael Ficarra

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new C(1 + a) A JavaScript Program

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new C ( 1 + a ) ▩ PUNCTUATOR ▩ KEYWORD ▩ IDENTIFIER ▩ NUMERIC Typical Tokenisation

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PARSER MAGIC

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Program body: [ ▢ ] NewExpression callee: ▢ arguments: [ ▢ ] Identifier name: "C" Identifier name: "a" Literal value: 1 BinaryExpression operator: "+" left: ▢ right: ▢ ExpressionStatement expression: ▢ new C(1 + a) Structured Representation (AST)

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Structured Representation (AST) { type: "Program" , body: [ { type: "ExpressionStatement" , expression: { type: "NewExpression" , callee: {type: "Identifier", name: "C"} , arguments: [ { type: "BinaryExpression" , operator: "+" , left: {type: "Literal", value: 1} , right: {type: "Identifier", name: "a"} } ]} } ]}

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Properties of a Good AST Format 1. each node tagged with its type(s) 2. nodes have no state or knowledge of context 3. disallows construction of invalid program 4. similar syntactic productions are meaningfully grouped

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Overly Permissive: Structures { type: "IfStatement", test: (...), consequent: { type: "IfStatement", test: (...), consequent: (...), alternate: null }, alternate: (...) } if(test) if(test) a(); else b(); { type: "TryStatement", block: (...), handler: null, guardedHandlers: [], finalizer: null } try { a() }

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Overly Permissive: Decl. Position

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Overly Permissive: List Properties 0, // needs to sequence at least 2 expressions var // needs at least one declarator // cannot contain more than one default switch(0) { default: 0 default: 0 }

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Overly Permissive: Context ● iteration context while(0) continue; while(0) break; ● switch context switch(0) { case 0: break; } ● function context (function(){ return; }); ● label set context label: label: 0; label: while(0) break label; label: while(0) continue label; label: (function(){ function f() { label: 0; } }); ● strict mode context function f(){ "use strict"; return {a: 0, a: 1}; };

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No DirectiveStatement Node (yet)

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Basic Tooling

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Static Analysis

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Evaluation

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Program Transformation

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Simplification Phase { type: "CallExpression" , callee: {type: "Identifier", name: "f"} , arguments: [ { type: "UnaryExpression" , prefix: true, operator: "!" , argument: { type: "UnaryExpression" , prefix: true, operator: "!" , argument: { type: "UnaryExpression" , prefix: true, operator: "!" , argument: {type: "Identifier", name: "a"} } } } ] } f(!!!a) f(!a)

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Expansion Phase { type: "MemberExpression" , computed: false , object: {type: "Identifier", name: "a"} , property: {type: "Identifier", name: "Infinity"} } a.Infinity a[1/0]

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Expansion Phase { type: "MemberExpression" , computed: true , object: {type: "Identifier", name: "a"} , property: { type: "BinaryExpression" , operator: "/" , left: {type: "Literal", value: 1, raw: "1"} , right: {type: "Literal", value: 0, raw: "0" } } } a.Infinity a[1/0]