Check if base visitor is defined (fix acornjs#829)

acorn-walk/README.md

@@ -124,3 +124,20 @@ the given position.
 **findNodeAfter**`(node, pos, test, base, state)` is similar to
 `findNodeAround`, but will match all nodes *after* the given position
 (testing outer nodes before inner nodes).
+
+
+## Walking with a plugin
+
+When you use a parser plugin, e.g. `acorn-jsx`, you might see an error
+message like `Base visitor for type JSXElement not found`.
+In such case, you need to pass an additional `base` parameter,
+in which you define the base visitors for all the types defined
+by the plugin. For example, you can extend the default base in the
+following way:
+
+```js
+walk.simple(parsedJsx, {}, {
+  ...walk.base,
+  JSXElement: () => {}
+})
+````

acorn-walk/src/index.js

@@ -1,5 +1,12 @@
 // AST walker module for Mozilla Parser API compatible trees
 
+function assertBaseVisitor(baseVisitor, type) {
+  if (typeof baseVisitor[type] !== "function") {
+    throw new Error(`Base visitor for type ${type} not found. You should define it in the 'base' parameter.`)
+  }
+  return true
+}
+
 // A simple walk is one where you simply specify callbacks to be
 // called on specific nodes. The last two arguments are optional. A
 // simple use would be
@@ -20,7 +27,7 @@ export function simple(node, visitors, baseVisitor, state, override) {
   if (!baseVisitor) baseVisitor = base
   ;(function c(node, st, override) {
     let type = override || node.type, found = visitors[type]
-    baseVisitor[type](node, st, c)
+    assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
     if (found) found(node, st)
   })(node, state, override)
 }
@@ -35,7 +42,7 @@ export function ancestor(node, visitors, baseVisitor, state) {
     let type = override || node.type, found = visitors[type]
     let isNew = node !== ancestors[ancestors.length - 1]
     if (isNew) ancestors.push(node)
-    baseVisitor[type](node, st, c)
+    assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
     if (found) found(node, st || ancestors, ancestors)
     if (isNew) ancestors.pop()
   })(node, state)
@@ -71,7 +78,7 @@ export function full(node, callback, baseVisitor, state, override) {
   if (!baseVisitor) baseVisitor = base
   ;(function c(node, st, override) {
     let type = override || node.type
-    baseVisitor[type](node, st, c)
+    assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
     if (!override) callback(node, st, type)
   })(node, state, override)
 }
@@ -85,7 +92,7 @@ export function fullAncestor(node, callback, baseVisitor, state) {
     let type = override || node.type
     let isNew = node !== ancestors[ancestors.length - 1]
     if (isNew) ancestors.push(node)
-    baseVisitor[type](node, st, c)
+    assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
     if (!override) callback(node, st || ancestors, ancestors, type)
     if (isNew) ancestors.pop()
   })(node, state)
@@ -102,7 +109,7 @@ export function findNodeAt(node, start, end, test, baseVisitor, state) {
       let type = override || node.type
       if ((start == null || node.start <= start) &&
           (end == null || node.end >= end))
-        baseVisitor[type](node, st, c)
+        assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
       if ((start == null || node.start === start) &&
           (end == null || node.end === end) &&
           test(type, node))
@@ -123,7 +130,7 @@ export function findNodeAround(node, pos, test, baseVisitor, state) {
     (function c(node, st, override) {
       let type = override || node.type
       if (node.start > pos || node.end < pos) return
-      baseVisitor[type](node, st, c)
+      assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
       if (test(type, node)) throw new Found(node, st)
     })(node, state)
   } catch (e) {
@@ -141,7 +148,7 @@ export function findNodeAfter(node, pos, test, baseVisitor, state) {
       if (node.end < pos) return
       let type = override || node.type
       if (node.start >= pos && test(type, node)) throw new Found(node, st)
-      baseVisitor[type](node, st, c)
+      assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
     })(node, state)
   } catch (e) {
     if (e instanceof Found) return e
@@ -159,7 +166,7 @@ export function findNodeBefore(node, pos, test, baseVisitor, state) {
     let type = override || node.type
     if (node.end <= pos && (!max || max.node.end < node.end) && test(type, node))
       max = new Found(node, st)
-    baseVisitor[type](node, st, c)
+    assertBaseVisitor(baseVisitor, type) && baseVisitor[type](node, st, c)
   })(node, state)
   return max
 }