Start on ES6 port

src/tokentype.js

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+// ## Token types
+
+// The assignment of fine-grained, information-carrying type objects
+// allows the tokenizer to store the information it has about a
+// token in a way that is very cheap for the parser to look up.
+
+// All token type variables start with an underscore, to make them
+// easy to recognize.
+
+// The `beforeExpr` property is used to disambiguate between regular
+// expressions and divisions. It is set on all token types that can
+// be followed by an expression (thus, a slash after them would be a
+// regular expression).
+//
+// `isLoop` marks a keyword as starting a loop, which is important
+// to know when parsing a label, in order to allow or disallow
+// continue jumps to that label.
+
+export class TokenType {
+  constructor(label, conf = {}) {
+    this.label = label
+    this.keyword = conf.keyword
+    this.beforeExpr = !!conf.beforeExpr
+    this.startsExpr = !!conf.startsExpr
+    this.isLoop = !!conf.isLoop
+    this.isAssign = !!conf.isAssign
+    this.prefix = !!conf.prefix
+    this.postfix = !!conf.postfix
+    this.binop = conf.binop || null
+    this.updateContext = null
+  }
+}
+
+function binop(name, prec) {
+  return new TokenType(name, {beforeExpr: true, binop: prec})
+}
+const beforeExpr = {beforeExpr: true}, startsExpr = {startsExpr: true}
+
+export const types = {
+  num: new TokenType("num", startsExpr),
+  regexp: new TokenType("regexp", startsExpr),
+  string: new TokenType("string", startsExpr),
+  name: new TokenType("name", startsExpr),
+  eof: new TokenType("eof"),
+
+  // Punctuation token types.
+  bracketL: new TokenType("[", {beforeExpr: true, startsExpr: true}),
+  bracketR: new TokenType("]"),
+  braceL: new TokenType("{", {beforeExpr: true, startsExpr: true}),
+  braceR: new TokenType("}"),
+  parenL: new TokenType("(", {beforeExpr: true, startsExpr: true}),
+  parenR: new TokenType(")"),
+  comma: new TokenType(",", beforeExpr),
+  semi: new TokenType(";", beforeExpr),
+  colon: new TokenType(":", beforeExpr),
+  dot: new TokenType("."),
+  question: new TokenType("?", beforeExpr),
+  arrow: new TokenType("=>", beforeExpr),
+  template: new TokenType("template"),
+  ellipsis: new TokenType("...", beforeExpr),
+  backQuote: new TokenType("`", startsExpr),
+  dollarBraceL: new TokenType("${", {beforeExpr: true, startsExpr: true}),
+
+  // Operators. These carry several kinds of properties to help the
+  // parser use them properly (the presence of these properties is
+  // what categorizes them as operators).
+  //
+  // `binop`, when present, specifies that this operator is a binary
+  // operator, and will refer to its precedence.
+  //
+  // `prefix` and `postfix` mark the operator as a prefix or postfix
+  // unary operator.
+  //
+  // `isAssign` marks all of `=`, `+=`, `-=` etcetera, which act as
+  // binary operators with a very low precedence, that should result
+  // in AssignmentExpression nodes.
+
+  eq: new TokenType("=", {beforeExpr: true, isAssign: true}),
+  assign: new TokenType("_=", {beforeExpr: true, isAssign: true}),
+  incDec: new TokenType("++/--", {prefix: true, postfix: true, startsExpr: true}),
+  prefix: new TokenType("prefix", {beforeExpr: true, prefix: true, startsExpr: true}),
+  logicalOR: binop("||", 1),
+  logicalAND: binop("&&", 2),
+  bitwiseOR: binop("|", 3),
+  bitwiseXOR: binop("^", 4),
+  bitwiseAND: binop("&", 5),
+  equality: binop("==/!=", 6),
+  relational: binop("</>", 7),
+  bitShift: binop("<</>>", 8),
+  plusMin: new TokenType("+/-", {beforeExpr: true, binop: 9, prefix: true, startsExpr: true}),
+  modulo: binop("%", 10),
+  star: binop("*", 10),
+  slash: binop("/", 10)
+}
+
+// Map keyword names to token types.
+
+export const keywords = {}
+
+// Succinct definitions of keyword token types
+function kw(name, options = {}) {
+  options.keyword = name
+  keywords[name] = types["_" + name] = new TokenType(name, options)
+}
+
+kw("break")
+kw("case", beforeExpr)
+kw("catch")
+kw("continue")
+kw("debugger")
+kw("default")
+kw("do", {isLoop: true})
+kw("else", beforeExpr)
+kw("finally")
+kw("for", {isLoop: true})
+kw("function")
+kw("if")
+kw("return", beforeExpr)
+kw("switch")
+kw("throw", beforeExpr)
+kw("try")
+kw("var")
+kw("let")
+kw("const")
+kw("while", {isLoop: true})
+kw("with")
+kw("new", {beforeExpr: true, startsExpr: true})
+kw("this", startsExpr)
+kw("super", startsExpr)
+kw("class")
+kw("extends", beforeExpr)
+kw("export")
+kw("import")
+kw("yield", {beforeExpr: true, startsExpr: true})
+kw("null", startsExpr)
+kw("true", startsExpr)
+kw("false", startsExpr)
+kw("in", {beforeExpr: true, binop: 7})
+kw("instanceof", {beforeExpr: true, binop: 7})
+kw("typeof", {beforeExpr: true, prefix: true, startsExpr: true})
+kw("void", {beforeExpr: true, prefix: true, startsExpr: true})
+kw("delete", {beforeExpr: true, prefix: true, startsExpr: true})

src/util.js

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+export function isArray(obj) {
+  return Object.prototype.toString.call(obj) === "[object Array]"
+}
+
+// Checks if an object has a property.
+
+export function has(obj, propName) {
+  return Object.prototype.hasOwnProperty.call(obj, propName)
+}