Typescript Expression Transformer

Expression trees for TypeScript similar to C#. This is the TypeScript transformer plugin.

Expression Type

type Expression<TType> = TType &
	{
		/**
		 * Compiled, executable, expression
		 */
		compiled?: TType;

		/**
		 * TypeScript expression tree
		 */
		expression?: ExpressionNode;

		/**
		 * Context variables
		 */
		context?: { [key: string]: any };
	};

This expression type is declared inside package which you need for runtime. It declares the Expression, ExpressionKind enum (taken from TypeScript) and maybe more runtime features in the future.

Example

An example demonstrating usage of member expression (PropertyAccessExpression) used to generate HTML Ids for model's elements. There is function fieldIdFor() which takes Expression argument. In TypeScript you set some arrow function as an argument (eg. m => m.foo). In transpile time, the transformer looks up all CallExpression of method/function declaration with parameter of type Expression<>. Then it takes the argument, creates the expression tree and replace the original expression with object (representing type Expression<>).

Get Started

Create project folder and run npm i typescript-expression-transformer ttypescript -D && npm i js-expr-tree

Source

src/field-id-for.ts

import {ExpressionKind} from "js-expr-tree"; // the runtime package of this transformer; it's a dependency

/**
 * Construct path from member expression
 * @param node
 * @param ignoredProperties
 * @param path
 */
function getPropertyPath(node, ignoredProperties: Array<string> = [], path: string = "") {
	if (!node) {
		return "";
	}

	let prefix = getPropertyPath(node.expression, ignoredProperties, path);
	return (prefix ? (prefix + "_") : "") + (node.name ? node.name.escapedText : "");
}

/**
 * Generate html id for model's property member expression
 * @param memberExpression
 */
export function fieldIdFor<TModel>(memberExpression: Expression<(m: TModel) => any>) {
	const expr = memberExpression.expression as ArrowFunctionExpressionNode;

	if (expr.kind != ExpressionKind.ArrowFunction) {
		throw new Error("Expression must be arrow function");
	}

	if (expr.body.kind != ExpressionKind.PropertyAccessExpression) {
		throw new Error("Arrow function body must be member expression without block body.");
	}

	const params = expr.parameters.map(p => p.name.escapedText);
	
	return getPropertyPath(expr.body, params);
}

This module exports the fieldIdFor function which returns the HTML id constructed by member expression tree.

Result Usage

Create random ts file with usage of fieldIdFor function.

index.ts

import {fieldIdFor} from "./src/field-id-for"

class Human {
	public name: string;
	public baz: { deepProp: string };
}

console.log("Generated id for property:", fieldIdFor<Human>(m => m.baz.deepProp));

Create typescript config file.

tsconfig.json

{
  "compilerOptions": {
    "module": "commonjs",
    "target": "esnext",
    "plugins": [
      {
        "transform": "typescript-expression-transformer"
      }
    ]
  },
  "exclude": [
    "node_modules"
  ]
}

Using ttypescript package, you can transpile source by running ttsc. index.js will be created.

Transpiled Code

index.js

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
var field_id_for_1 = require("./field-id-for");
class Human {
}
console.log("Generated id for property:", field_id_for_1.fieldIdFor({ compiled: m => m.baz.deepProp, context: { Human }, expression: { "flags": 128, "kind": 197, "parameters": [{ "flags": 0, "kind": 151, "name": { "flags": 0, "escapedText": "m", "flowNode": { "flags": 2 } }, "symbol": { "flags": 1, "escapedName": "m", "declarations": [null], "exports": {} } }], "equalsGreaterThanToken": { "flags": 0, "kind": 37 }, "body": { "flags": 0, "kind": 189, "expression": { "flags": 0, "kind": 189, "expression": { "flags": 0, "escapedText": "m" }, "name": { "flags": 0, "escapedText": "baz" } }, "name": { "flags": 0, "escapedText": "deepProp" } }, "flowNode": { "flags": 2 }, "symbol": { "flags": 16, "escapedName": "__function", "declarations": [null] }, "locals": {} } }));

Run node index.js and you'll see the result.

Result

Generated id for property: baz_deepProp

Tip

For work with AST you can use https://astexplorer.net which prints tree nicely. Use the TypeScript AST.