added tests

cypress/fixtures/example.json

@@ -0,0 +1,5 @@
+{
+  "name": "Using fixtures to represent data",
+  "email": "hello@cypress.io",
+  "body": "Fixtures are a great way to mock data for responses to routes"
+}

cypress/integration/console.spec.js

@@ -14,3 +14,32 @@ describe('draw a vector', () => {
         cy.get('#2').invoke('attr','d').should('eq','M550 350 L500 200');
     })
 })
+
+describe('draw a lazy vector', () => {
+    it('adds the svg vector', () => {
+        cy.visit('http://localhost:8080');
+
+        cy.get('#command_input').type("a = vector(0,0,0.5,0.5){enter}");
+        cy.get('#0').invoke('attr','d').should('eq','M550 350 L600 300');
+        cy.get('#0').invoke('attr','class').should('eq','vector');
+        cy.get('#0').invoke('attr','marker-end').should('eq','url(#arrow)');
+
+        cy.get('#command_input').type("b = vector(0,0,-1,1){enter}");
+        cy.get('#1').invoke('attr','d').should('eq','M550 350 L450 250');
+
+        cy.get('#command_input').type("c = \"a + b\"{enter}"); // same as above, but lazy
+        cy.get('#3').invoke('attr','d').should('eq','M550 350 L500 200');
+
+        cy.get('#command_input').type("a = a*2{enter}"); // so what happens to lazy, when it's constituents are altered?
+        // a is now double in size, so c must also changed
+        // c now refers to @4
+        cy.get('#4').invoke('attr','d').should('eq','M550 350 L650 250');
+
+        cy.get('#command_input').type("a = 1{enter}"); // so what happens to lazy, when it's constituents are altered?
+
+        cy.get("#l4").then(text => {
+            expect(text.text()).to.equal('@4');   // binding has been changed (to 1, ie not a vector),
+                                                        // so this is now a 'free' vector with a reference label
+        });
+    })
+})

cypress/support/index.js

@@ -1,2 +1,3 @@
 // import './commands'
 
+require('cypress-xpath');

package-lock.json

@@ -1643,6 +1643,12 @@
         }
       }
     },
+    "cypress-xpath": {
+      "version": "1.6.2",
+      "resolved": "https://registry.npmjs.org/cypress-xpath/-/cypress-xpath-1.6.2.tgz",
+      "integrity": "sha512-mtwJPl840GQPGtb480fKR5vDIcijBHhAVwby5/AIPIT/UVT7UJhM2L42/R+venR7N01I0PoOJErb6UiMbCyUxg==",
+      "dev": true
+    },
     "dashdash": {
       "version": "1.14.1",
       "resolved": "https://registry.npmjs.org/dashdash/-/dashdash-1.14.1.tgz",

package.json

@@ -22,6 +22,7 @@
   "devDependencies": {
     "css-loader": "^5.0.2",
     "cypress": "^6.5.0",
+    "cypress-xpath": "^1.6.2",
     "eslint": "^7.20.0",
     "eslint-loader": "^4.0.2",
     "eslint-plugin-cypress": "^2.11.2",

src/js/index.js

@@ -37,17 +37,18 @@ const show = function (vector) {
 }
 
 export const update_lazy_objects = function () {
-    let b = Object.values(bindings);
-    for (let i = 0; i < b.length; i++) {
+    let b = Object.values(bindings);                // a lazy expression must be bound
+    for (let i = 0; i < b.length; i++) {            // unbound ones can exist, but are meaningless, because you cannot refer to them
         let binding = b[i];
-        if (state[binding.name].lazy_expression) {
-            let value = visit(state[binding.name].lazy_expression);
-            let existing_value = bindings[binding.name].evaluated;
+        if (state[binding.name].lazy_expression) {                  // if lazy,
+            let value = visit(state[binding.name].lazy_expression); // reevaluate,
+            let existing_value = bindings[binding.name].evaluated;  // update view
             if (existing_value.id) {
+                // update view after reevaluation of lazy vectors
                 update_vector_arrow(existing_value.id, value);
                 bindings[binding.name].evaluated = value;
             } else if (value.is_new && value.is_vector){
-                // hidden lazy object reappearing
+                // hidden lazy vector reappears
                 value.label_text=binding.name;
                 add_vector_arrow_to_svg(value);
             }