Calculate Simple

calculateSimple.js

const expression1 = '6+9-12';
const expression2 = '1+2-3+4-5+6-7';
const expression3 = '100+200+300';
const expression4 = '1-2-3-0';
const expression5 = '255';
const expression6 = '0-1-2-3';

function calculate(input) {
  return reduceOperations(buildOperations(input));
}

function buildOperations(input) {
  const characters = input.split('');
  const operations = [];
  let operation = [];
  for (let characterIndex = 0; characterIndex < characters.length; characterIndex++) {
    const character = characters[characterIndex];
    if (['+', '-'].includes(character)) {
      operations.push(operation.join(''));
      operations.push(character);
      operation = [];
    } else {
      operation.push(character);
    }
  }
  operations.push(operation.join(''));
  return operations;
}

function reduceOperations(operations) {
  return operations.slice(2).reduce((output, operation) => {
    if (['+', '-'].includes(operation)) {
      output.operator = operation;
    } else {
      if (output.operator === '+') {
        output.result += parseInt(operation, 10);
      }
      if (output.operator === '-') {
        output.result -= parseInt(operation, 10);
      }
    }
    return output;
  }, {
    result: parseInt(operations[0], 10),
    operator: operations[1],
  }).result;
}

console.log(calculate(expression1) === 3);
console.log(calculate(expression2) === -2);
console.log(calculate(expression3) === 600);
console.log(calculate(expression4) === -4);
console.log(calculate(expression5) === 255);
console.log(calculate(expression6) === -6);