feat: ignore function definition image (#215)

* feat: ignore functions with images
* chore: refactor logic
* refactor: allow type narrowing before isNodeFunction
* refactor: remove comment

closes #212

src/index.ts

@@ -156,19 +156,32 @@ export class EsbuildServerlessPlugin implements ServerlessPlugin {
     return Boolean(runtimeMatcher?.[runtime]);
   }
 
+  /**
+   * Checks if the function has a handler
+   * @param {Serverless.FunctionDefinitionHandler | Serverless.FunctionDefinitionImage} func the function to be checked
+   * @returns {boolean} true if the function has a handler
+   */
+  private isFunctionDefinitionHandler(
+    func: Serverless.FunctionDefinitionHandler | Serverless.FunctionDefinitionImage
+  ): func is Serverless.FunctionDefinitionHandler {
+    return Boolean((func as Serverless.FunctionDefinitionHandler)?.handler);
+  }
+
   get functions(): Record<string, Serverless.FunctionDefinitionHandler> {
+    let functions: typeof this.serverless.service.functions;
     if (this.options.function) {
-      //only return the function if it's a node function:
-      const func = this.serverless.service.getFunction(this.options.function) as Serverless.FunctionDefinitionHandler;
-      return this.isNodeFunction(func) ? { [this.options.function]: func } : {};
+      functions = {
+        [this.options.function]: this.serverless.service.getFunction(this.options.function),
+      };
+    } else {
+      functions = this.serverless.service.functions;
     }
 
     // ignore all functions with a different runtime than nodejs:
     const nodeFunctions: Record<string, Serverless.FunctionDefinitionHandler> = {};
-    const functions = this.serverless.service.functions;
-    for(const funcName in functions) {
+    for (const funcName in functions) {
       const func = functions[funcName] as Serverless.FunctionDefinitionHandler;
-      if (this.isNodeFunction(func)) {
+      if (this.isFunctionDefinitionHandler(func) && this.isNodeFunction(func)) {
         nodeFunctions[funcName] = func;
       }
     }