code: @Override
    protected void visitLetValueNode(LetValueNode node) {
  
      String generatedVarName = node.getUniqueVarName();
  
      // Generate code to define the local var.
      Expression value = translateExpr(node.getExpr());
      jsCodeBuilder.append(VariableDeclaration.builder(generatedVarName).setRhs(value).build());
  
      // Add a mapping for generating future references to this local var.
      templateTranslationContext
          .soyToJsVariableMappings()
          .put(node.getVarName(), id(generatedVarName));
    }
docs: Example:
  
  <pre>
  {let $boo: $foo.goo[$moo] /}
  </pre>
  
  might generate
  
  <pre>
  var boo35 = opt_data.foo.goo[opt_data.moo];
  </pre>