Added program for infix to postfix conversion using stack

Data-Structures/Stack/ToPostfix.js

@@ -0,0 +1,85 @@
+/*
+ * Author: Harinath-B (https://github.com/Harinath-B)
+ * Infix and POstfix notation explanation can be found in the following links:
+ * Wikipedia: https://en.wikipedia.org/wiki/Infix_notation / https://en.wikipedia.org/wiki/Reverse_Polish_notation
+ */
+
+class Stack {
+  constructor () {
+    this.stack = []
+    this.top = -1
+  }
+
+  // Adds a value to the end of the Stack
+  push (newValue) {
+    this.stack.push(newValue)
+    this.top++
+  }
+
+  // Returns and removes the last element of the Stack
+  pop () {
+    if (this.top !== -1) {
+      this.top--
+      return this.stack.pop()
+    }
+    throw new Error('Stack Underflow')
+  }
+
+  // Returns the number of elements in the Stack
+  length () {
+    return this.top
+  }
+
+  // Returns true if stack is empty, false otherwise
+  isEmpty () {
+    return this.top === -1
+  }
+
+  // Returns the last element without removing it
+  last () {
+    if (this.top !== -1) {
+      return this.stack[this.length()]
+    }
+    return null
+  }
+}
+
+const isAlNum = (c) => {
+  return c.match(/^[a-z0-9]+$/i) !== null
+}
+
+const priority = (op) => {
+  if (op === '+' || op === '-') {
+    return 1
+  } else if (op === '*' || op === '/') {
+    return 2
+  }
+  return 0
+}
+
+function ToPostfix (infix) {
+  let postfix = ''
+  const opStack = new Stack()
+  for (const c of infix) {
+    if (isAlNum(c)) {
+      postfix += c
+    } else if (c === '(') {
+      opStack.push(c)
+    } else if (c === ')') {
+      let x = ''
+      while ((x = opStack.pop()) !== '(') {
+        postfix += x
+      }
+    } else {
+      while (priority(opStack.last()) > priority(c)) {
+        postfix += opStack.pop()
+      } opStack.push(c)
+    }
+  }
+  while (opStack.top !== -1) {
+    postfix += opStack.pop()
+  }
+  return postfix
+}
+
+export { ToPostfix }

Data-Structures/Stack/test/ToPostfix.test.js

@@ -0,0 +1,17 @@
+import { ToPostfix } from '../ToPostfix'
+
+describe('ToPostfix', () => {
+  it('Converting the given infix expression to postfix', () => {
+    const infix = 'a*b+(c-d/e)'
+    const postfix = ToPostfix(infix)
+    const expected = 'ab*cde/-+'
+    expect(postfix).toEqual(expected)
+  })
+
+  it('Testing empty input', () => {
+    const infix = ''
+    const postfix = ToPostfix(infix)
+    const expected = ''
+    expect(postfix).toEqual(expected)
+  })
+})