A simple four arithmetic operations implementation

caculator.js

const Decimal = require('decimal.js');

// 计算表达式的值
function calculate(expression) {
  const words = splitExpression(expression);

  const outputs = [];
  const signs = [];
  while (words.length) {
    const word = words.shift();

    if (!isNaN(word)) {
      outputs.push(new Decimal(word));
      continue;
    }

    while (signs.length) {
      const sign = signs.pop();
      if (!higher(word, sign)) {
        outputs.push(sign)
      } else {
        signs.push(sign);
        break
      }
    }
    signs.push(word)
  }
  while (signs.length) {
    outputs.push(signs.pop())
  }

  const result = [];
  outputs.forEach(word => {
    if (isNaN(word)) {
      const right = result.pop();
      const left = result.pop();
      result.push(binaryOperate(left, word, right))
    } else {
      result.push(word)
    }
  });

  return result.pop();
}

// 把表达式分割为 运算符 和 数字
// 示例：
//    splitExpression("-1*-3+-2") 的结果为 ["-1", "*", "-3", "+", "-2"]
function splitExpression(expression) {
  const words = expression.split(/((?<!\d)[+-]?\d*\.?\d*)/);
  return words.filter(word => word)
}

// 判断 sign1 的优先级是否比 sign2 高
function higher(sign1, sign2) {
  return ["*", "/"].includes(sign1) && ["-", "+"].includes(sign2);
}


// 二元运算，包括加减乘除
function binaryOperate(left, operator, right) {
  switch (operator) {
    case "+":
      return left.plus(right);
    case "-":
      return left.minus(right);
    case "*":
      return left.times(right);
    case "/":
      return left.div(right);
  }
}

// 测试
const assert = require('assert');
assert.equal(calculate("1+2"), 3);
assert.equal(calculate("1-2"), -1);
assert.equal(calculate("1*2"), 2);
assert.equal(calculate("1/2"), 0.5);

assert.equal(calculate("1+2/2/4"), 1.25);
assert.equal(calculate("1*2+2/4"), 2.5);

assert.equal(calculate("1*2*3+2/2/4*1-2"), 4.25);

assert.equal(calculate("-1*-3+-2"), 1);
assert.equal(calculate("1*2*3+2/2/4*1-2"), 4.25);
assert.equal(calculate("-1*-3+-2/-1"), 5);
assert.equal(calculate("-1*-2*-3-4"), -10);

assert.equal(calculate("0.1+0.1"), 0.2);
assert.equal(calculate(".1+.1"), 0.2);
assert.equal(calculate("0.+.1"), 0.1);
assert.equal(calculate("0.1+0.2"), 0.3);
assert.equal(calculate("0.1+0.1+0.1+0.2+0.3"), 0.8);