// ─── install ───
// npm install express body-parser expr-eval
const express = require('express');
const bodyParser = require('body-parser');
const { Parser } = require('expr-eval');
const app = express();
app.use(bodyParser.json());
// Only digits, space, operators, decimal point and parentheses:
const SAFE_EXPR = /^[0-9+\-*/^().\s]+$/;
app.post('/calculate', (req, res) => {
const expr = (req.body.expression || '').trim();
// 1) Reject empty or disallowed input
if (!expr || !SAFE_EXPR.test(expr)) {
return res.status(400).json({ error: 'Invalid characters in expression.' });
}
try {
// 2) Safely parse & evaluate
const parser = new Parser({ operators: { // lock down functions:
add: true, subtract: true,
multiply: true, divide: true,
power: true
}});
const result = parser.evaluate(expr);
// 3) Return numeric result only
if (typeof result !== 'number' || !isFinite(result)) {
throw new Error('Non-finite result');
}
res.json({ result });
} catch (err) {
res.status(400).json({ error: 'Malformed expression.' });
}
});
app.listen(3000, () => console.log('Calculator server listening on :3000'));
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