add and use error classes for undefined variables

cleans up funky message-matching code in expression.ts for lenient.

src/context/context.ts

@@ -3,6 +3,7 @@ import { __assign } from 'tslib'
 import { NormalizedFullOptions, defaultOptions } from '../liquid-options'
 import { Scope } from './scope'
 import { isArray, isNil, isString, isFunction, toLiquid } from '../util/underscore'
+import { InternalUndefinedVariableError } from '../util/error'
 
 export class Context {
   private scopes: Scope[] = [{}]
@@ -42,7 +43,7 @@ export class Context {
     return paths.reduce((scope, path) => {
       scope = readProperty(scope, path)
       if (isNil(scope) && this.opts.strictVariables) {
-        throw new TypeError(`undefined variable: ${path}`)
+        throw new InternalUndefinedVariableError(path)
       }
       return scope
     }, scope)

src/render/expression.ts

@@ -12,6 +12,7 @@ import { Context } from '../context/context'
 import { range, toValue } from '../util/underscore'
 import { Tokenizer } from '../parser/tokenizer'
 import { operatorImpls } from '../render/operator'
+import { UndefinedVariableError, InternalUndefinedVariableError } from '../util/error'
 
 export class Expression {
   private operands: any[] = []
@@ -49,12 +50,10 @@ export function evalToken (token: Token | undefined, ctx: Context, lenient: bool
     try {
       return ctx.get([variable, ...props])
     } catch (e) {
-      // for lenient, we catch the error thrown by Context.getFromScope() for undefined vars.
+      if (lenient && e instanceof InternalUndefinedVariableError) {
-      // Alt, we could make this more robust by setting a flag or using a separate error class.
-      if (lenient && e instanceof TypeError && e.message.startsWith("undefined variable:")) {
         return null
       } else {
-        throw(e)
+        throw(new UndefinedVariableError(e, token))
       }
     }
   }

src/util/error.ts

@@ -48,6 +48,27 @@ export class RenderError extends LiquidError {
   }
 }
 
+export class UndefinedVariableError extends LiquidError {
+  public constructor (err: Error, token: Token) {
+    super(err, token)
+    this.name = 'UndefinedVariableError'
+    this.message = err.message
+    super.update()
+  }
+}
+
+// only used internally; raised where we don't have token information,
+// so it can't be an UndefinedVariableError.
+export class InternalUndefinedVariableError extends Error {
+  variableName: string
+
+  public constructor (variableName: string) {
+    super(`undefined variable: ${variableName}`)
+    this.name = 'InternalUndefinedVariableError'
+    this.variableName = variableName
+  }
+}
+
 export class AssertionError extends Error {
   public constructor (message: string) {
     super(message)