Using createVariableStatement in a compiler transformer breaks the compiler

[pedro-pedrosa]

TypeScript Version: 2.8.0-dev.20180307

Search Terms: createVariableStatement const

Code (compiler)

// A *self-contained* demonstration of the problem follows...
import * as ts from 'typescript';

function transformer(program: ts.Program): ts.TransformerFactory<ts.SourceFile> {
    return (context: ts.TransformationContext) => (file: ts.SourceFile) => transformFile(program, context, file);
}

function transformFile(program: ts.Program, context: ts.TransformationContext, file: ts.SourceFile): ts.SourceFile {
    const transformedFile = ts.visitEachChild(file, child => visit(child, context, file), context);
    return transformedFile;
}
function visit(node: ts.Node, context: ts.TransformationContext, file: ts.SourceFile): ts.Node {
    if (ts.isMethodDeclaration(node)) {
        const newNode = ts.createMethod(
            [ts.createToken(ts.SyntaxKind.StaticKeyword)], 
            [], 
            null, 
            node.name, 
            null, 
            [], 
            node.parameters, 
            node.type, 
            ts.createBlock([ 
                ts.createVariableStatement(
                    [ts.createToken(ts.SyntaxKind.ConstKeyword)], 
                    [ts.createVariableDeclaration('myConst', null, ts.createLiteral('value'))]
                ),
                ...(node.body ? node.body.statements : [])
            ])
        );
        console.log(ts.createPrinter().printNode(ts.EmitHint.Unspecified, newNode, file));
        return newNode;
    }
    return ts.visitEachChild(node, child => visit(child, context, file), context);
}

const program = ts.createProgram([
  '../transformer-issue-src/src/A.ts'
], {
    target: ts.ScriptTarget.ES5,
    module: ts.ModuleKind.CommonJS,
    moduleResolution: ts.ModuleResolutionKind.NodeJs,
    importHelpers: true,
    alwaysStrict: true,
    noImplicitAny: true,
    noImplicitThis: true,
    removeComments: true,
    sourceMap: true,
    outDir: "../transformer-issue-src/lib",
    declaration: true,
    declarationDir: "../transformer-issue-src/lib",
    lib: [
      "lib.es2017.d.ts",
    ],
    experimentalDecorators: true,
    noEmitOnError: true,
});

const transformers = {
  before: [
    transformer(program),
  ]
}
const result = program.emit(undefined, undefined, undefined, false, transformers);

Code (test program, ../transformer-issue-src/src/A.ts)

export class A {
    static myMethod() {
        return 'value';
    }
}

Expected behavior:
The .js file should be emitted.
The console.log statement should print the following:

static myMethod() { const myConst = "value"; return 'value'; }

Actual behavior:
The .js file is not emitted due to a compiler exception. Message and stack trace:

TypeError: Cannot read property 'transformFlags' of null
    at aggregateTransformFlagsForNode (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:54626:18)
    at Object.aggregateTransformFlags (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:54611:9)
    at visitNode (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:53754:12)
    at Object.visitEachChild (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:54053:108)
    at visitVariableDeclaration (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:61625:30)
    at Object.flatMap (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:2047:25)
    at visitVariableDeclarationList (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:61510:39)
    at visitJavaScript (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:60282:28)
    at visitor (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:60243:24)
    at visitNode (d:\solutions\transformer-issue-compiler\node_modules\typescript\lib\typescript.js:53755:23)

Even if I remove the ts.createToken(ts.SyntaxKind.ConstKeyword) token, this exception is still thrown.

Also, the console.log statement prints the following (the var keyword is left there):

static myMethod() { const var myConst = "value"; return 'value'; }

[RyanCavanaugh]

80% sure the problem is that you're passing null instead of undefined for some of those parameters. The TS API basically never uses null and will behave badly if given it because it's only ever checking for === undefined

[pedro-pedrosa]

Replacing the null in createVariableDeclaration with undefined made the exception go away (wasn't needed on createMethod), however it still prints const var.

[RyanCavanaugh]

Pass NodeFlags.Const to createVariableDeclarationList rather than trying to push in a const keyword in the array.

There are lots of examples in the TypeScript codebase that show calling this function correctly.

[pedro-pedrosa]

That worked, I have a correct output now.

Thank you.