handle this.get special case in helpers

src/validate/js/propValidators/helpers.js

@@ -24,6 +24,12 @@ export default function helpers ( validator, prop ) {
 				}
 
 				else if ( lexicalDepth === 0 ) {
+					// handle special case that's caused some people confusion — using `this.get(...)` instead of passing argument
+					// TODO do the same thing for computed values?
+					if ( node.type === 'CallExpression' && node.callee.type === 'MemberExpression' && node.callee.object.type === 'ThisExpression' && node.callee.property.name === 'get' && !node.callee.property.computed ) {
+						validator.error( `Cannot use this.get(...) — it must be passed into the helper function as an argument`, node.start );
+					}
+
 					if ( node.type === 'ThisExpression' ) {
 						validator.error( `Helpers should be pure functions — they do not have access to the component instance and cannot use 'this'. Did you mean to put this in 'methods'?`, node.start );
 					}

test/validator/samples/helper-purity-check-no-this/errors.json

@@ -1,8 +1,8 @@
 [{
 	"message": "Helpers should be pure functions — they do not have access to the component instance and cannot use 'this'. Did you mean to put this in 'methods'?",
-	"pos": 74,
+	"pos": 95,
 	"loc": {
 		"line": 7,
-		"column": 11
+		"column": 4
 	}
 }]

test/validator/samples/helper-purity-check-no-this/input.html

@@ -1,10 +1,10 @@
-{{foo()}}
+<button on:click='foo()'>foo</button>
 
 <script>
 	export default {
 		helpers: {
 			foo () {
-				return this.get( 'bar' );
+				this.set({ foo: true });
 			}
 		}
 	}

test/validator/samples/helper-purity-check-this-get/errors.json

@@ -0,0 +1,8 @@
+[{
+	"message": "Cannot use this.get(...) — it must be passed into the helper function as an argument",
+	"pos": 74,
+	"loc": {
+		"line": 7,
+		"column": 11
+	}
+}]

test/validator/samples/helper-purity-check-this-get/input.html

@@ -0,0 +1,11 @@
+{{foo()}}
+
+<script>
+	export default {
+		helpers: {
+			foo () {
+				return this.get( 'bar' );
+			}
+		}
+	}
+</script>