Expression trees

import { parse, ConstantNode } from 'mathjs'

// Filter an expression tree
console.log('Filter all symbol nodes "x" in the expression "x^2 + x/4 + 3*y"')
const node = parse('x^2 + x/4 + 3*y')
const filtered = node.filter(function (node) {
  return node.isSymbolNode && node.name === 'x'
})
// returns an array with two entries: two SymbolNodes 'x'

filtered.forEach(function (node) {
  console.log(node.type, node.toString())
})
// outputs:
//  SymbolNode x
//  SymbolNode x

// Traverse an expression tree
console.log()
console.log('Traverse the expression tree of expression "3 * x + 2"')
const node1 = parse('3 * x + 2')
node1.traverse(function (node, path, parent) {
  switch (node.type) {
    case 'OperatorNode':
      console.log(node.type, node.op)
      break
    case 'ConstantNode':
      console.log(node.type, node.value)
      break
    case 'SymbolNode':
      console.log(node.type, node.name)
      break
    default: console.log(node.type)
  }
})
// outputs:
//   OperatorNode +
//   OperatorNode *
//   ConstantNode 3
//   SymbolNode x
//   ConstantNode 2

// transform an expression tree
console.log()
console.log('Replace all symbol nodes "x" in expression "x^2 + 5*x" with a constant 3')
const node2 = parse('x^2 + 5*x')
const transformed = node2.transform(function (node, path, parent) {
  if (node.isSymbolNode && node.name === 'x') {
    return new ConstantNode(3)
  } else {
    return node
  }
})
console.log(transformed.toString())
// outputs: '3 ^ 2 + 5 * 3'