EvaluateExpression.js

/**
 * Evaluate a numeric operations string in postfix notation using a stack.
 * Supports basic arithmetic operations: +, -, *, /
 * @see https://www.geeksforgeeks.org/evaluation-of-postfix-expression/
 * @param {string} expression - Numeric operations expression to evaluate. Must be a valid postfix expression.
 * @returns {number|null} - Result of the expression evaluation, or null if the expression is invalid.
 */
function evaluatePostfixExpression(expression) {
 const stack = []

 // Helper function to perform an operation and push the result to the stack. Returns success.
 function performOperation(operator) {
 const rightOp = stack.pop() // Right operand is the top of the stack
 const leftOp = stack.pop() // Left operand is the next item on the stack

 if (leftOp === undefined || rightOp === undefined) {
 return false // Invalid expression
 }
 switch (operator) {
 case '+':
 stack.push(leftOp + rightOp)
 break
 case '-':
 stack.push(leftOp - rightOp)
 break
 case '*':
 stack.push(leftOp * rightOp)
 break
 case '/':
 if (rightOp === 0) {
 return false
 }
 stack.push(leftOp / rightOp)
 break
 default:
 return false // Unknown operator
 }
 return true
 }

 const tokens = expression.split(/\s+/)

 for (const token of tokens) {
 if (!isNaN(parseFloat(token))) {
 // If the token is a number, push it to the stack
 stack.push(parseFloat(token))
 } else {
 // If the token is an operator, perform the operation
 if (!performOperation(token)) {
 return null // Invalid expression
 }
 }
 }

 return stack.length === 1 ? stack[0] : null
}

export { evaluatePostfixExpression }