Build a CLI Expression Evaluator

In this tutorial, you'll learn how to use the samasya library to create a command-line application that:

• Evaluates mathematical expressions.
• Allows users to define and manage variables.
• Adds custom functions dynamically.
• Saves evaluation history for later review.

By the end of this tutorial, you'll have a fully functional CLI Expression Evaluator that demonstrates the power and flexibility of the samasya library.

tip

This Project Heavily Uses the Evaluator Class, please read it's documentation for more information here

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Step 1: Setting Up the Project

1. Create a new directory for your project:

mkdir cli-expression-evaluator
cd cli-expression-evaluator

2. Initialize the project:

npm init -y

3. Install dependencies:

npm install samasya readline-sync

4. Set up the project structure:

cli-expression-evaluator/
├── index.js
├── variables.json
├── functions.json
├── history.json

• index.js: Main application file.
• variables.json: Stores user-defined variables.
• functions.json: Stores user-defined functions.
• history.json: Logs the evaluation history.

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Step 2: Writing the code

Copy the following code into your index.js file:

const readline = require('readline-sync');
const fs = require('fs');
const { Evaluator } = require('samasya');

// File paths
const variablesFile = './variables.json';
const functionsFile = './functions.json';
const historyFile = './history.json';

// Load predefined variables and functions
let variables = {};
if (fs.existsSync(variablesFile)) {
  variables = JSON.parse(fs.readFileSync(variablesFile, 'utf8'));
}

let customFunctions = {};
if (fs.existsSync(functionsFile)) {
  customFunctions = JSON.parse(fs.readFileSync(functionsFile, 'utf8'));
}

// Initialize Evaluator
const evaluator = new Evaluator(variables);

// Load custom functions into the evaluator
for (const [name, funcBody] of Object.entries(customFunctions)) {
  evaluator.setFunction(name, eval(funcBody));
}

// Function to save evaluation history
function saveHistory(expression, result) {
  let history = [];
  if (fs.existsSync(historyFile)) {
    history = JSON.parse(fs.readFileSync(historyFile, 'utf8'));
  }
  history.push({ expression, result, timestamp: new Date().toISOString() });
  fs.writeFileSync(historyFile, JSON.stringify(history, null, 2), 'utf8');
}

// Function to add a variable
function addVariable(name, value) {
  evaluator.setVariable(name, value);
  variables[name] = value;
  fs.writeFileSync(variablesFile, JSON.stringify(variables, null, 2), 'utf8');
  console.log(`Variable "${name}" set to ${value}`);
}

// Function to add a custom function
function addCustomFunction(name, funcBody) {
  try {
    const func = eval(funcBody); // Create a function from the body
    evaluator.setFunction(name, func);
    customFunctions[name] = funcBody;
    fs.writeFileSync(functionsFile, JSON.stringify(customFunctions, null, 2), 'utf8');
    console.log(`Function "${name}" added successfully.`);
  } catch (error) {
    console.error(`Failed to add function: ${error.message}`);
  }
}

// Main CLI loop
while (true) {
  console.log('\n--- Command-Line Expression Evaluator ---');
  console.log('1. Evaluate an expression');
  console.log('2. Add a variable');
  console.log('3. Add a custom function');
  console.log('4. View evaluation history');
  console.log('5. Exit');

  const choice = readline.question('Enter your choice: ');

  switch (choice) {
    case '1':
      const expression = readline.question('Enter the expression: ');
      try {
        const result = evaluator.evaluate(expression);
        console.log(`Result: ${result}`);
        saveHistory(expression, result);
      } catch (error) {
        console.error(`Error: ${error.message}`);
      }
      break;

    case '2':
      const varName = readline.question('Enter variable name: ');
      const varValue = parseFloat(readline.question('Enter variable value: '));
      addVariable(varName, varValue);
      break;

    case '3':
      const funcName = readline.question('Enter function name: ');
      const funcBody = readline.question('Enter the function body (e.g., (a, b) => a + b): ');
      addCustomFunction(funcName, funcBody);
      break;

    case '4':
      if (fs.existsSync(historyFile)) {
        const history = JSON.parse(fs.readFileSync(historyFile, 'utf8'));
        console.log('\nEvaluation History:');
        history.forEach((entry, index) => {
          console.log(`${index + 1}. ${entry.expression} = ${entry.result}`);
        });
      } else {
        console.log('No history found.');
      }
      break;

    case '5':
      console.log('Goodbye!');
      process.exit();

    default:
      console.log('Invalid choice. Please try again.');
  }
}

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Step 3: Run the Program

1. Run the program:

node index.js

2. Follow the on-screen prompts to evaluate expressions, add variables, and create custom functions.

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Step 4: Download the Example Project

To save time you can also download the entire project as a zip file below.

Download Zip

Unzip the project and then install the necessary dependencies.

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