main.js

 function doubleAll(numbers) {
  const doubled = [];

  for (let i = 0; i < numbers.length; i++) {
    doubled.push(numbers[i] * 2);
  }

  return doubled;
}

function yelledGreetings(greetings) {
  const yelled = [];

  for (let i = 0; i < greetings.length; i++) {
    yelled.push(greetings[i] + '!')
  }

  return yelled;
}

function absoluteValues(numbers) {
  const absolutes = [];

  for (let i = 0; i < numbers.length; i++) {
    const number = numbers[i]
    if (number < 0) {
      absolutes.push(number * -1);
    } else {
      absolutes.push(number);
    }
  }

  return absolutes;
}

// or, instead of an if/else, we could use Math.abs to get the absolute value:
function absoluteValues2(numbers) {
  const absolutes = [];

  for (let i = 0; i < numbers.length; i++) {
    absolutes.push(Math.abs(numbers[i]));
  }

  return absolutes;
}

function upperCaseFirstLetters(names) {
  let uppercasedNames = [];

  for (let i = 0; i < names.length; i++) {
    uppercasedNames.push(names[i][0].toUpperCase() + names[i].slice(1).toLowerCase())
  }

  return uppercasedNames;
}

// intermediate variables version
function changeToInitials(names) {
  const initialsList = [];

  for (let i = 0; i < names.length; i++) {
    const name = names[i];
    const initial1 = name[0];
    const spaceIndex = name.indexOf(' ');
    const initial2 = name[spaceIndex + 1];

    initialsList.push(initial1 + initial2);
  }

  return initialsList;
}

// with a crazy one-liner instead of intermediate variables
function changeToInitials2(names) {
  const initialsList = [];

  for (let i = 0; i < names.length; i++) {
    initialsList.push(names[i][0] + names[i][names[i].indexOf(' ') + 1]);
  }

  return initialsList;
}

// internal for loop version (string building)
function changeToInitials3(names) {
  const initialsList = [];

  for (let i = 0; i < names.length; i++) {
    let initials = '';

    for (let i = 0; i < name.length; i++) {
      if (i === 0 || name[i - 1] === ' ') {
        initials = initials + name[i].toUpperCase();
      }
    }

    initialsList.push(initials)
  }

  return initialsList;
}

// if version
function doubleOdd(numbers) {
  const answers = [];

  for (let i = 0; i < numbers.length; i++) {
    if (numbers[i] % 2 === 1 || numbers[i] % 2 === -1) {
      answers.push(numbers[i] * 2)
    } else {
      answers.push(numbers[i])
    }
  }

  return answers;
}

// if and re-assignment version:
function doubleOdd2(numbers) {
  const answers = [];

  for (let i = 0; i < numbers.length; i++) {
    let answer = numbers[i];
    if (answer % 2 === 1 || answer % 2 === -1) {
      answer = answer * 2;
    }
    
    answers.push(answer);
  }

  return answers;
}

// Math.abs (absolute value) version
function doubleOdd3(numbers) {
  const answers = [];

  for (let i = 0; i < numbers.length; i++) {
    if (Math.abs(numbers[i]) % 2 === 1) {
      answers.push(numbers[i] * 2);
    } else {
      answers.push(numbers[i]);
    }
  }

  return answers;
}

function add1ToLeft(numbers) {
  const with1Addeds = [];
  for (let i = 0; i < numbers.length; i++) {
    const stringifiedNumber = numbers[i].toString();
    if (numbers[i] < 0) {
      const stringWith1Added = '-1' + stringifiedNumber.slice(1)
      with1Addeds.push(Number(stringWith1Added));
    } else {
      const stringWith1Added = '1' + stringifiedNumber;
      with1Addeds.push(Number(stringWith1Added));
    }
  }

  return with1Addeds;
}


if (typeof doubleAll === 'undefined') {
  doubleAll = undefined;
}

if (typeof yelledGreetings === 'undefined') {
  yelledGreetings = undefined;
}

if (typeof absoluteValues === 'undefined') {
  absoluteValues = undefined;
}

if (typeof upperCaseFirstLetters === 'undefined') {
  upperCaseFirstLetters = undefined;
}

if (typeof changeToInitials === 'undefined') {
  changeToInitials = undefined;
}

if (typeof doubleOdd === 'undefined') {
  doubleOdd = undefined;
}

if (typeof add1ToLeft === 'undefined') {
  add1ToLeft = undefined;
}


module.exports = {
  doubleAll,
  yelledGreetings,
  absoluteValues,
  upperCaseFirstLetters,
  changeToInitials,
  doubleOdd,
  add1ToLeft,
}