While there are many JavaScript libraries for building solutions for a wide range of problems, it’s not easy for novices to harness their power. We show how TypeScript, a gradually typed superset of JavaScript, can be used to bridge the gap between JavaScript and Blockly, a framework for creating block-based programming environments that greatly reduces the potential for syntax and semantic errors. In particular, we define a mapping from TypeScript to Blockly that makes it simple to create a domain-specific Blockly editor for a JavaScript library via a TypeScript declaration file. This mapping is supported by Microsoft MakeCode (https://makecode.com). An online editor for exploring the mapping from TypeScript to Blockly is at https://makecode.com/playground.