Custom Transform (Plugin ) System

import * as ts from ‘typescript’

export default function(/*opts?: Opts*/) {

  function visitor(ctx: ts.TransformationContext, sf: ts.SourceFile) {

    const visitor: ts.Visitor = (node: ts.Node): ts.VisitResult => {

      // here we can check each node and potentially return 

      // new nodes if we want to leave the node as is, and 

      // continue searching through child nodes:

      return ts.visitEachChild(node, visitor, ctx)

    }

    return visitor

  }

  return (ctx: ts.TransformationContext): ts.Transformer => {

    return (sf: ts.SourceFile) => ts.visitNode(sf, visitor(ctx, sf))

  }

}

const visitor: ts.Visitor = (node: ts.Node): ts.VisitResult => {

  if (ts.isCallExpression(node) &&

      node.expression.getText(sf) == 'safely') {

    // get the argument to safely

    const target = node.arguments[0]

    // check to make sure it is a property access, like "a.b"

    if (ts.isPropertyAccessExpression(target) {

      // return a binary expression with a && a.b

      return ts.createBinary(

        target.expression, // the left hand operand is the object

        ts.SyntaxKind.AmpersandAmpersandToken, // the && operator

        target) // the right hand operand is the full expression

    }

  }

  // otherwise continue visiting all the nodes

  return ts.visitEachChild(node, visitor, ctx)

}