init

public/calculators/interest.js

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+import {currency, revive, persist, labelInput, labelSelect} from '/js/util.js';
+export default {
+  id:'interest', name:'Interest (Simple & Compound)', about:'Compute simple or compound interest with flexible compounding and contributions.',
+  render(root){
+    const key='calc_interest_v1';
+    const s = revive(key,{principal:1000, rate:5, years:3, compound:'12', contrib:0, contribFreq:'12'});
+    const ui = document.createElement('div');
+    ui.append(
+      labelInput('Principal','number','principal', s.principal,{step:'0.01',min:'0'}),
+      labelInput('Annual rate (%)','number','rate', s.rate,{step:'0.0001',min:'0'}),
+      labelSelect('Compounding','compound', s.compound, [['1','Yearly'],['4','Quarterly'],['12','Monthly'],['365','Daily'],['0','Simple (no compounding)']]),
+      labelInput('Years','number','years', s.years,{step:'0.1',min:'0'}),
+      labelInput('Recurring contribution (per period below)','number','contrib', s.contrib,{step:'0.01',min:'0'}),
+      labelSelect('Contribution frequency','contribFreq', s.contribFreq, [['1','Yearly'],['4','Quarterly'],['12','Monthly']])
+    );
+    const out = document.createElement('div'); out.className='result'; ui.append(out);
+
+    function calc(){
+      const P = +ui.querySelector('[name=principal]').value||0;
+      const r = (+ui.querySelector('[name=rate]').value||0)/100;
+      const years = +ui.querySelector('[name=years]').value||0;
+      const n = +ui.querySelector('[name=compound]').value; // 0 => simple
+      const A = +ui.querySelector('[name=contrib]').value||0;
+      const f = +ui.querySelector('[name=contribFreq]').value||1;
+
+      let future=0, interest=0;
+      if(n===0){
+        interest = P * r * years;
+        const contribs = A * f * years;
+        future = P + interest + contribs;
+      }else{
+        const periods = n * years;
+        const i = r / n;
+        future = P * Math.pow(1+i, periods);
+        if(A>0){
+          const eff = Math.pow(1+i, n/f) - 1;
+          const m = Math.round(periods * f / n);
+          future += A * ((Math.pow(1+eff, m) - 1) / eff);
+        }
+        interest = future - P - (A>0?A*Math.round(n*years * f / n):0);
+      }
+      out.innerHTML = `
+        <div><strong>Future value:</strong> ${currency(future)}</div>
+        <div class="muted">Estimated interest earned: ${currency(Math.max(0,interest))}</div>
+      `;
+      persist(key,{principal:P, rate:r*100, years, compound:String(n), contrib:A, contribFreq:String(f)});
+    }
+    ui.addEventListener('input', calc); calc(); root.append(ui);
+  }
+}