Added forward euler in js, fixed ODE

Author: Ariana1729

contents/forward_euler_method/code/javascript/euler.js

@@ -0,0 +1,35 @@
+function solveEuler(timeStep, n){
+  const result=[];
+  if (n != 0) {
+    result[0] = 1;
+    for (let i = 1; i < n; ++i) {
+      result[i] = result[i-1] - 3 * result[i-1] * timeStep;
+    }
+  } 
+  return result;
+}
+
+function checkResult(result, threshold, timeStep) {
+  let approx = true;
+  for (let i = 0; i < result.length; ++i){
+    const solution = Math.exp(-3 * i * timeStep);
+    if (Math.abs(result[i] - solution) > threshold) {
+      console.log(result[i]+" "+solution);
+      approx = false;
+    }
+  }
+  return approx;
+}
+
+const timeStep = 0.01;
+const n = 100;
+const threshold = 0.01;
+
+const result = solveEuler(timeStep, n)
+const approx = checkResult(result, threshold, timeStep)
+
+if (approx) {
+  console.log("All values within threshold");
+}else{
+  console.log("Value(s) not in threshold");
+}

contents/forward_euler_method/forward_euler_method.md

@@ -95,7 +95,7 @@ That said, variations of this method *are* certainly used (for example Crank-Nic
 
 Like in the case of [Verlet Integration](../verlet_integration/verlet_integration.md), the easiest way to test to see if this method works is to test it against a simple test-case.
 Here, the most obvious test-case would be dropping a ball from 5 meters, which is my favorite example, but proved itself to be slightly less enlightening than I would have thought.
-So, this time, let's remove ourselves from any physics and instead solve the following ODE: $$y(t)' = -3t$$ with the initial condition that $$y(0) = 1$$.
+So, this time, let's remove ourselves from any physics and instead solve the following ODE: $$y(t)' = -3y$$ with the initial condition that $$y(0) = 1$$.
 Note that in this case, the velocity is directly given by the ODE and the acceleration is not part of the model.
 
 {% method %}
@@ -126,6 +126,8 @@ Full code for the visualization follows:
 [import, lang:"fortran"](code/fortran/euler.f90)
 {% sample lang="go" %}
 [import, lang:"go"](code/golang/euler.go)
+{% sample lang="js" %}
+[import, lang:"javascript"](code/javascript/euler.js)
 {% endmethod %}
 
 <script>