node-utils.js

/**
 * @fileoverview Utilities for finding and converting TSNodes into ESTreeNodes
 * @author James Henry <https://github.com/JamesHenry>
 * @copyright jQuery Foundation and other contributors, https://jquery.org/
 * MIT License
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const ts = require("typescript"),
    unescape = require("lodash.unescape");

//------------------------------------------------------------------------------
// Private
//------------------------------------------------------------------------------

const SyntaxKind = ts.SyntaxKind;

const ASSIGNMENT_OPERATORS = [
    SyntaxKind.EqualsToken,
    SyntaxKind.PlusEqualsToken,
    SyntaxKind.MinusEqualsToken,
    SyntaxKind.AsteriskEqualsToken,
    SyntaxKind.SlashEqualsToken,
    SyntaxKind.PercentEqualsToken,
    SyntaxKind.LessThanLessThanEqualsToken,
    SyntaxKind.GreaterThanGreaterThanEqualsToken,
    SyntaxKind.GreaterThanGreaterThanGreaterThanEqualsToken,
    SyntaxKind.AmpersandEqualsToken,
    SyntaxKind.BarEqualsToken,
    SyntaxKind.CaretEqualsToken
];

const LOGICAL_OPERATORS = [
    SyntaxKind.BarBarToken,
    SyntaxKind.AmpersandAmpersandToken
];

const TOKEN_TO_TEXT = {};
TOKEN_TO_TEXT[SyntaxKind.OpenBraceToken] = "{";
TOKEN_TO_TEXT[SyntaxKind.CloseBraceToken] = "}";
TOKEN_TO_TEXT[SyntaxKind.OpenParenToken] = "(";
TOKEN_TO_TEXT[SyntaxKind.CloseParenToken] = ")";
TOKEN_TO_TEXT[SyntaxKind.OpenBracketToken] = "[";
TOKEN_TO_TEXT[SyntaxKind.CloseBracketToken] = "]";
TOKEN_TO_TEXT[SyntaxKind.DotToken] = ".";
TOKEN_TO_TEXT[SyntaxKind.DotDotDotToken] = "...";
TOKEN_TO_TEXT[SyntaxKind.SemicolonToken] = ";";
TOKEN_TO_TEXT[SyntaxKind.CommaToken] = ",";
TOKEN_TO_TEXT[SyntaxKind.LessThanToken] = "<";
TOKEN_TO_TEXT[SyntaxKind.GreaterThanToken] = ">";
TOKEN_TO_TEXT[SyntaxKind.LessThanEqualsToken] = "<=";
TOKEN_TO_TEXT[SyntaxKind.GreaterThanEqualsToken] = ">=";
TOKEN_TO_TEXT[SyntaxKind.EqualsEqualsToken] = "==";
TOKEN_TO_TEXT[SyntaxKind.ExclamationEqualsToken] = "!=";
TOKEN_TO_TEXT[SyntaxKind.EqualsEqualsEqualsToken] = "===";
TOKEN_TO_TEXT[SyntaxKind.InstanceOfKeyword] = "instanceof";
TOKEN_TO_TEXT[SyntaxKind.ExclamationEqualsEqualsToken] = "!==";
TOKEN_TO_TEXT[SyntaxKind.EqualsGreaterThanToken] = "=>";
TOKEN_TO_TEXT[SyntaxKind.PlusToken] = "+";
TOKEN_TO_TEXT[SyntaxKind.MinusToken] = "-";
TOKEN_TO_TEXT[SyntaxKind.AsteriskToken] = "*";
TOKEN_TO_TEXT[SyntaxKind.AsteriskAsteriskToken] = "**";
TOKEN_TO_TEXT[SyntaxKind.SlashToken] = "/";
TOKEN_TO_TEXT[SyntaxKind.PercentToken] = "%";
TOKEN_TO_TEXT[SyntaxKind.PlusPlusToken] = "++";
TOKEN_TO_TEXT[SyntaxKind.MinusMinusToken] = "--";
TOKEN_TO_TEXT[SyntaxKind.LessThanLessThanToken] = "<<";
TOKEN_TO_TEXT[SyntaxKind.LessThanSlashToken] = "</";
TOKEN_TO_TEXT[SyntaxKind.GreaterThanGreaterThanToken] = ">>";
TOKEN_TO_TEXT[SyntaxKind.GreaterThanGreaterThanGreaterThanToken] = ">>>";
TOKEN_TO_TEXT[SyntaxKind.AmpersandToken] = "&";
TOKEN_TO_TEXT[SyntaxKind.BarToken] = "|";
TOKEN_TO_TEXT[SyntaxKind.CaretToken] = "^";
TOKEN_TO_TEXT[SyntaxKind.ExclamationToken] = "!";
TOKEN_TO_TEXT[SyntaxKind.TildeToken] = "~";
TOKEN_TO_TEXT[SyntaxKind.AmpersandAmpersandToken] = "&&";
TOKEN_TO_TEXT[SyntaxKind.BarBarToken] = "||";
TOKEN_TO_TEXT[SyntaxKind.QuestionToken] = "?";
TOKEN_TO_TEXT[SyntaxKind.ColonToken] = ":";
TOKEN_TO_TEXT[SyntaxKind.EqualsToken] = "=";
TOKEN_TO_TEXT[SyntaxKind.PlusEqualsToken] = "+=";
TOKEN_TO_TEXT[SyntaxKind.MinusEqualsToken] = "-=";
TOKEN_TO_TEXT[SyntaxKind.AsteriskEqualsToken] = "*=";
TOKEN_TO_TEXT[SyntaxKind.AsteriskAsteriskEqualsToken] = "**=";
TOKEN_TO_TEXT[SyntaxKind.SlashEqualsToken] = "/=";
TOKEN_TO_TEXT[SyntaxKind.PercentEqualsToken] = "%=";
TOKEN_TO_TEXT[SyntaxKind.LessThanLessThanEqualsToken] = "<<=";
TOKEN_TO_TEXT[SyntaxKind.GreaterThanGreaterThanEqualsToken] = ">>=";
TOKEN_TO_TEXT[SyntaxKind.GreaterThanGreaterThanGreaterThanEqualsToken] = ">>>=";
TOKEN_TO_TEXT[SyntaxKind.AmpersandEqualsToken] = "&=";
TOKEN_TO_TEXT[SyntaxKind.BarEqualsToken] = "|=";
TOKEN_TO_TEXT[SyntaxKind.CaretEqualsToken] = "^=";
TOKEN_TO_TEXT[SyntaxKind.AtToken] = "@";
TOKEN_TO_TEXT[SyntaxKind.InKeyword] = "in";
TOKEN_TO_TEXT[SyntaxKind.UniqueKeyword] = "unique";
TOKEN_TO_TEXT[SyntaxKind.KeyOfKeyword] = "keyof";
TOKEN_TO_TEXT[SyntaxKind.NewKeyword] = "new";
TOKEN_TO_TEXT[SyntaxKind.ImportKeyword] = "import";

/**
 * Find the first matching child based on the given sourceFile and predicate function.
 * @param {TSNode} node The current TSNode
 * @param {Object} sourceFile The full AST source file
 * @param {Function} predicate The predicate function to apply to each checked child
 * @returns {TSNode|undefined} a matching child TSNode
 */
function findFirstMatchingChild(node, sourceFile, predicate) {
    const children = node.getChildren(sourceFile);
    for (let i = 0; i < children.length; i++) {
        const child = children[i];
        if (child && predicate(child)) {
            return child;
        }

        const grandChild = findFirstMatchingChild(child, sourceFile, predicate);
        if (grandChild) {
            return grandChild;
        }
    }
    return undefined;
}

//------------------------------------------------------------------------------
// Public
//------------------------------------------------------------------------------

module.exports = {
    /**
     * Expose the enum of possible TSNode `kind`s.
     */
    SyntaxKind,
    isAssignmentOperator,
    isLogicalOperator,
    getTextForTokenKind,
    isESTreeClassMember,
    hasModifier,
    isComma,
    getBinaryExpressionType,
    getLocFor,
    getLoc,
    isToken,
    isJSXToken,
    getDeclarationKind,
    getTSNodeAccessibility,
    hasStaticModifierFlag,
    findNextToken,
    findFirstMatchingToken,
    findChildOfKind,
    findFirstMatchingAncestor,
    findAncestorOfKind,
    hasJSXAncestor,
    unescapeStringLiteralText,
    isComputedProperty,
    isOptional,
    fixExports,
    getTokenType,
    convertToken,
    convertTokens,
    getNodeContainer,
    isWithinTypeAnnotation,
    isTypeKeyword,
    isComment,
    isJSDocComment,
    createError
};

/**
 * Returns true if the given TSToken is the assignment operator
 * @param  {TSToken}  operator the operator token
 * @returns {boolean}          is assignment
 */
function isAssignmentOperator(operator) {
    return ASSIGNMENT_OPERATORS.indexOf(operator.kind) > -1;
}

/**
 * Returns true if the given TSToken is a logical operator
 * @param  {TSToken}  operator the operator token
 * @returns {boolean}          is a logical operator
 */
function isLogicalOperator(operator) {
    return LOGICAL_OPERATORS.indexOf(operator.kind) > -1;
}

/**
 * Returns the string form of the given TSToken SyntaxKind
 * @param  {number}  kind the token's SyntaxKind
 * @returns {string}          the token applicable token as a string
 */
function getTextForTokenKind(kind) {
    return TOKEN_TO_TEXT[kind];
}

/**
 * Returns true if the given TSNode is a valid ESTree class member
 * @param  {TSNode}  node TypeScript AST node
 * @returns {boolean}      is valid ESTree class member
 */
function isESTreeClassMember(node) {
    return node.kind !== SyntaxKind.SemicolonClassElement;
}

/**
 * Checks if a TSNode has a modifier
 * @param {SyntaxKind} modifierKind TypeScript SyntaxKind modifier
 * @param {TSNode} node TypeScript AST node
 * @returns {boolean} has the modifier specified
 */
function hasModifier(modifierKind, node) {
    return !!node.modifiers && !!node.modifiers.length && node.modifiers.some(modifier => modifier.kind === modifierKind);
}

/**
 * Returns true if the given TSToken is a comma
 * @param  {TSToken}  token the TypeScript token
 * @returns {boolean}       is comma
 */
function isComma(token) {
    return token.kind === SyntaxKind.CommaToken;
}

/**
 * Returns true if the given TSNode is a comment
 * @param {TSNode} node the TypeScript node
 * @returns {boolean} is commment
 */
function isComment(node) {
    return node.kind === SyntaxKind.SingleLineCommentTrivia || node.kind === SyntaxKind.MultiLineCommentTrivia;
}

/**
 * Returns true if the given TSNode is a JSDoc comment
 * @param {TSNode} node the TypeScript node
 * @returns {boolean} is JSDoc comment
 */
function isJSDocComment(node) {
    return node.kind === SyntaxKind.JSDocComment;
}

/**
 * Returns the binary expression type of the given TSToken
 * @param  {TSToken} operator the operator token
 * @returns {string}          the binary expression type
 */
function getBinaryExpressionType(operator) {
    if (isAssignmentOperator(operator)) {
        return "AssignmentExpression";
    } else if (isLogicalOperator(operator)) {
        return "LogicalExpression";
    }
    return "BinaryExpression";
}

/**
 * Returns line and column data for the given start and end positions,
 * for the given AST
 * @param  {Object} start start data
 * @param  {Object} end   end data
 * @param  {Object} ast   the AST object
 * @returns {Object}       the loc data
 */
function getLocFor(start, end, ast) {
    const startLoc = ast.getLineAndCharacterOfPosition(start),
        endLoc = ast.getLineAndCharacterOfPosition(end);

    return {
        start: {
            line: startLoc.line + 1,
            column: startLoc.character
        },
        end: {
            line: endLoc.line + 1,
            column: endLoc.character
        }
    };
}

/**
 * Returns line and column data for the given ESTreeNode or ESTreeToken,
 * for the given AST
 * @param  {ESTreeToken|ESTreeNode} nodeOrToken the ESTreeNode or ESTreeToken
 * @param  {Object} ast         the AST object
 * @returns {Object}             the loc data
 */
function getLoc(nodeOrToken, ast) {
    return getLocFor(nodeOrToken.getStart(), nodeOrToken.end, ast);
}

/**
 * Returns true if a given TSNode is a token
 * @param  {TSNode} node the TSNode
 * @returns {boolean}     is a token
 */
function isToken(node) {
    return node.kind >= SyntaxKind.FirstToken && node.kind <= SyntaxKind.LastToken;
}

/**
 * Returns true if a given TSNode is a JSX token
 * @param  {TSNode} node TSNode to be checked
 * @returns {boolean}       is a JSX token
 */
function isJSXToken(node) {
    return (
        node.kind >= SyntaxKind.JsxElement &&
        node.kind <= SyntaxKind.JsxAttribute
    );
}

/**
 * Returns true if the given TSNode.kind value corresponds to a type keyword
 * @param {number} kind TypeScript SyntaxKind
 * @returns {boolean} is a type keyword
 */
function isTypeKeyword(kind) {
    switch (kind) {
        case SyntaxKind.AnyKeyword:
        case SyntaxKind.BooleanKeyword:
        case SyntaxKind.NeverKeyword:
        case SyntaxKind.NumberKeyword:
        case SyntaxKind.ObjectKeyword:
        case SyntaxKind.StringKeyword:
        case SyntaxKind.SymbolKeyword:
        case SyntaxKind.UnknownKeyword:
        case SyntaxKind.VoidKeyword:
            return true;
        default:
            return false;
    }
}

/**
 * Returns the declaration kind of the given TSNode
 * @param  {TSNode}  node TypeScript AST node
 * @returns {string}     declaration kind
 */
function getDeclarationKind(node) {
    switch (node.kind) {
        case SyntaxKind.TypeAliasDeclaration:
            return "type";
        case SyntaxKind.VariableDeclarationList:
            if (node.flags & ts.NodeFlags.Let) {
                return "let";
            }
            if (node.flags & ts.NodeFlags.Const) {
                return "const";
            }
            return "var";
        default:
            throw "Unable to determine declaration kind.";
    }
}

/**
 * Gets a TSNode's accessibility level
 * @param {TSNode} node The TSNode
 * @returns {string | null} accessibility "public", "protected", "private", or null
 */
function getTSNodeAccessibility(node) {
    const modifiers = node.modifiers;
    if (!modifiers) {
        return null;
    }
    for (let i = 0; i < modifiers.length; i++) {
        const modifier = modifiers[i];
        switch (modifier.kind) {
            case SyntaxKind.PublicKeyword:
                return "public";
            case SyntaxKind.ProtectedKeyword:
                return "protected";
            case SyntaxKind.PrivateKeyword:
                return "private";
            default:
                continue;
        }
    }
    return null;
}

/**
 * Returns true if the given TSNode has the modifier flag set which corresponds
 * to the static keyword.
 * @param {TSNode} node The TSNode
 * @returns {boolean} whether or not the static modifier flag is set
 */
function hasStaticModifierFlag(node) {
    /**
     * TODO: Remove dependency on private TypeScript method
     */
    return Boolean(ts.getModifierFlags(node) & ts.ModifierFlags.Static);
}

/**
 * Finds the next token based on the previous one and its parent
 * @param {TSToken} previousToken The previous TSToken
 * @param {TSNode} parent The parent TSNode
 * @returns {TSToken} the next TSToken
 */
function findNextToken(previousToken, parent) {
    /**
     * TODO: Remove dependency on private TypeScript method
     */
    return ts.findNextToken(previousToken, parent);
}

/**
 * Find the first matching token based on the given predicate function.
 * @param {TSToken} previousToken The previous TSToken
 * @param {TSNode} parent The parent TSNode
 * @param {Function} predicate The predicate function to apply to each checked token
 * @returns {TSToken|undefined} a matching TSToken
 */
function findFirstMatchingToken(previousToken, parent, predicate) {
    while (previousToken) {
        if (predicate(previousToken)) {
            return previousToken;
        }
        previousToken = findNextToken(previousToken, parent);
    }
    return undefined;
}

/**
 * Finds the first child TSNode which matches the given kind
 * @param {TSNode} node The parent TSNode
 * @param {number} kind The TSNode kind to match against
 * @param {Object} sourceFile The full AST source file
 * @returns {TSNode|undefined} a matching TSNode
 */
function findChildOfKind(node, kind, sourceFile) {
    return findFirstMatchingChild(node, sourceFile, child => child.kind === kind);
}

/**
 * Find the first matching ancestor based on the given predicate function.
 * @param {TSNode} node The current TSNode
 * @param {Function} predicate The predicate function to apply to each checked ancestor
 * @returns {TSNode|undefined} a matching parent TSNode
 */
function findFirstMatchingAncestor(node, predicate) {
    while (node) {
        if (predicate(node)) {
            return node;
        }
        node = node.parent;
    }
    return undefined;
}

/**
 * Finds the first parent TSNode which mastches the given kind
 * @param {TSNode} node The current TSNode
 * @param {number} kind The TSNode kind to match against
 * @returns {TSNode|undefined} a matching parent TSNode
 */
function findAncestorOfKind(node, kind) {
    return findFirstMatchingAncestor(node, parent => parent.kind === kind);
}


/**
 * Returns true if a given TSNode has a JSX token within its hierarchy
 * @param  {TSNode} node TSNode to be checked
 * @returns {boolean}       has JSX ancestor
 */
function hasJSXAncestor(node) {
    return !!findFirstMatchingAncestor(node, isJSXToken);
}

/**
 * Unescape the text content of string literals, e.g. &amp; -> &
 * @param {string} text The escaped string literal text.
 * @returns {string} The unescaped string literal text.
 */
function unescapeStringLiteralText(text) {
    return unescape(text);
}

/**
 * Returns true if a given TSNode is a computed property
 * @param  {TSNode} node TSNode to be checked
 * @returns {boolean}       is Computed Property
 */
function isComputedProperty(node) {
    return node.kind === SyntaxKind.ComputedPropertyName;
}

/**
 * Returns true if a given TSNode is optional (has QuestionToken)
 * @param  {TSNode} node TSNode to be checked
 * @returns {boolean}       is Optional
 */
function isOptional(node) {
    return (node.questionToken)
        ? (node.questionToken.kind === SyntaxKind.QuestionToken) : false;
}

/**
 * Returns true if the given TSNode is within the context of a "typeAnnotation",
 * which effectively means - is it coming from its parent's `type` or `types` property
 * @param  {TSNode} node TSNode to be checked
 * @returns {boolean}       is within "typeAnnotation context"
 */
function isWithinTypeAnnotation(node) {
    return node.parent.type === node || (node.parent.types && node.parent.types.indexOf(node) > -1);
}

/**
 * Fixes the exports of the given TSNode
 * @param  {TSNode} node   the TSNode
 * @param  {Object} result result
 * @param  {Object} ast    the AST
 * @returns {TSNode}        the TSNode with fixed exports
 */
function fixExports(node, result, ast) {
    // check for exports
    if (node.modifiers && node.modifiers[0].kind === SyntaxKind.ExportKeyword) {
        const exportKeyword = node.modifiers[0],
            nextModifier = node.modifiers[1],
            lastModifier = node.modifiers[node.modifiers.length - 1],
            declarationIsDefault = nextModifier && (nextModifier.kind === SyntaxKind.DefaultKeyword),
            varToken = findNextToken(lastModifier, ast);

        result.range[0] = varToken.getStart();
        result.loc = getLocFor(result.range[0], result.range[1], ast);

        const declarationType = declarationIsDefault ? "ExportDefaultDeclaration" : "ExportNamedDeclaration";

        const newResult = {
            type: declarationType,
            declaration: result,
            range: [exportKeyword.getStart(), result.range[1]],
            loc: getLocFor(exportKeyword.getStart(), result.range[1], ast)
        };

        if (!declarationIsDefault) {
            newResult.specifiers = [];
            newResult.source = null;
        }

        return newResult;

    }

    return result;
}

/**
 * Returns the type of a given ESTreeToken
 * @param  {ESTreeToken} token the ESTreeToken
 * @returns {string}       the token type
 */
function getTokenType(token) {
    // Need two checks for keywords since some are also identifiers
    if (token.originalKeywordKind) {

        switch (token.originalKeywordKind) {
            case SyntaxKind.NullKeyword:
                return "Null";

            case SyntaxKind.GetKeyword:
            case SyntaxKind.SetKeyword:
            case SyntaxKind.TypeKeyword:
            case SyntaxKind.ModuleKeyword:
                return "Identifier";

            default:
                return "Keyword";
        }
    }

    if (token.kind >= SyntaxKind.FirstKeyword && token.kind <= SyntaxKind.LastFutureReservedWord) {
        if (token.kind === SyntaxKind.FalseKeyword || token.kind === SyntaxKind.TrueKeyword) {
            return "Boolean";
        }

        return "Keyword";
    }

    if (token.kind >= SyntaxKind.FirstPunctuation && token.kind <= SyntaxKind.LastBinaryOperator) {
        return "Punctuator";
    }

    if (token.kind >= SyntaxKind.NoSubstitutionTemplateLiteral && token.kind <= SyntaxKind.TemplateTail) {
        return "Template";
    }

    switch (token.kind) {
        case SyntaxKind.NumericLiteral:
            return "Numeric";

        case SyntaxKind.JsxText:
            return "JSXText";

        case SyntaxKind.StringLiteral:

            // A TypeScript-StringLiteral token with a TypeScript-JsxAttribute or TypeScript-JsxElement parent,
            // must actually be an ESTree-JSXText token
            if (token.parent && (token.parent.kind === SyntaxKind.JsxAttribute || token.parent.kind === SyntaxKind.JsxElement)) {
                return "JSXText";
            }

            return "String";

        case SyntaxKind.RegularExpressionLiteral:
            return "RegularExpression";

        case SyntaxKind.Identifier:
        case SyntaxKind.ConstructorKeyword:
        case SyntaxKind.GetKeyword:
        case SyntaxKind.SetKeyword:

            // falls through
        default:
    }

    // Some JSX tokens have to be determined based on their parent
    if (token.parent) {
        if (token.kind === SyntaxKind.Identifier && token.parent.kind === SyntaxKind.PropertyAccessExpression && hasJSXAncestor(token)) {
            return "JSXIdentifier";
        }

        if (isJSXToken(token.parent)) {
            if (token.kind === SyntaxKind.PropertyAccessExpression) {
                return "JSXMemberExpression";
            }

            if (token.kind === SyntaxKind.Identifier) {
                return "JSXIdentifier";
            }
        }
    }

    return "Identifier";
}

/**
 * Extends and formats a given ESTreeToken, for a given AST
 * @param  {ESTreeToken} token the ESTreeToken
 * @param  {Object} ast   the AST object
 * @returns {ESTreeToken}       the converted ESTreeToken
 */
function convertToken(token, ast) {
    const start = (token.kind === SyntaxKind.JsxText) ? token.getFullStart() : token.getStart(),
        end = token.getEnd(),
        value = ast.text.slice(start, end),
        newToken = {
            type: getTokenType(token),
            value,
            range: [start, end],
            loc: getLocFor(start, end, ast)
        };

    if (newToken.type === "RegularExpression") {
        newToken.regex = {
            pattern: value.slice(1, value.lastIndexOf("/")),
            flags: value.slice(value.lastIndexOf("/") + 1)
        };
    }

    return newToken;
}

/**
 * Converts all tokens for the given AST
 * @param  {Object} ast the AST object
 * @returns {ESTreeToken[]}     the converted ESTreeTokens
 */
function convertTokens(ast) {
    const result = [];
    /**
     * @param  {TSNode} node the TSNode
     * @returns {undefined}
     */
    function walk(node) {
        // TypeScript generates tokens for types in JSDoc blocks. Comment tokens
        // and their children should not be walked or added to the resulting tokens list.
        if (isComment(node) || isJSDocComment(node)) {
            return;
        }

        if (isToken(node) && node.kind !== SyntaxKind.EndOfFileToken) {
            const converted = convertToken(node, ast);

            if (converted) {
                result.push(converted);
            }
        } else {
            node.getChildren().forEach(walk);
        }
    }
    walk(ast);
    return result;
}

/**
 * Get container token node between range
 * @param  {Object} ast the AST object
 * @param {int} start The index at which the comment starts.
 * @param {int} end The index at which the comment ends.
 * @returns {TSToken}       typescript container token
 * @private
 */
function getNodeContainer(ast, start, end) {
    let container = null;

    /**
     * @param  {TSNode} node the TSNode
     * @returns {undefined}
     */
    function walk(node) {
        const nodeStart = node.pos;
        const nodeEnd = node.end;

        if (start >= nodeStart && end <= nodeEnd) {
            if (isToken(node)) {
                container = node;
            } else {
                node.getChildren().forEach(walk);
            }
        }
    }
    walk(ast);

    return container;
}

/**
 * @param {Object} ast     the AST object
 * @param {int} start      the index at which the error starts
 * @param {string} message the error message
 * @returns {Object}       converted error object
 */
function createError(ast, start, message) {
    const loc = ast.getLineAndCharacterOfPosition(start);
    return {
        index: start,
        lineNumber: loc.line + 1,
        column: loc.character,
        message
    };
}