JavaScript: forEach, map, Arrow Functions, setTimeout, setInterval, filter, some, every, and reduce

forEach Method

forEach is used to iterate over arrays. Here is a simple example:

const numbers = [1, 2, 3, 4, 5, 6, 7];

numbers.forEach(function (el) {
  if (el % 2 === 0) {
    console.log(el);
  }
});

Now let's use forEach with an array of objects:

const movies = [
  {
    title: 'Indiana Jones',
    score: 77,
  },
  {
    title: 'Star Trek',
    score: 94,
  },
  {
    title: 'Deadpool',
    score: 79,
  },
];

movies.forEach(function (movie) {
  console.log(`${movie.title} - ${movie.score}/100`);
});

Map

Map creates a new array with the results from calling a callback on every element in the array.

First, let's iterate over a basic array:

const lCase = ['jim', 'bob', 'abby'];
const uCase = lCase.map(function (t) {
  return t.toUpperCase();
});

console.log(lCase, uCase);

Now, let's iterate over an array of objects:

const movies = [
  {
    title: 'Indiana Jones',
    score: 77,
  },
  {
    title: 'Star Trek',
    score: 94,
  },
  {
    title: 'Deadpool',
    score: 79,
  },
];

const titles = movies.map(function (movie) {
  return movie.title;
});
console.log(titles);

Arrow Functions

We can write arrow functions as follows to simplify our syntax:

const square = (x) => {
  return x * x;
};

const sum = (x, y) => {
  return x + y;
};

const rollDie = () => {
  return Math.floor(Math.random() * 6) + 1;
};

console.log(square(2), sum(2, 3));
console.log(rollDie());

We can also re-visit the movie example and use an arrow function:

const movies = [
  {
    title: 'Indiana Jones',
    score: 77,
  },
  {
    title: 'Star Trek',
    score: 94,
  },
  {
    title: 'Deadpool',
    score: 79,
  },
];

const newMovies = movies.map((movie) => {
  return `${movie.title} - ${movie.score} / 10`;
});

Implicit Returns

Implicit returns are another shorthand way of writing functions, some examples include:

const rollDie = () => Math.floor(Math.random() * 6) + 1;
const isEven = (num) => num % 2 === 0;

Set Timeout

Runs the callback after x milliseconds

console.log('Apears 1st');
// takes callback, then milliseconds
setTimeout(() => {
  console.log('Apears 3rd');
}, 3000);

console.log('Apears 2nd');

Set Interval

Repeats the callback every x milliseconds

setInterval(() => {
  console.log(Math.random());
}, 2000);

If we want to eventually make the callback stop, we can do this:

const id = setInterval(() => {
  console.log(Math.random());
}, 2000);

clearInterval(id); // stops the loop

Filter Method

Filter creates a new array with the elements that return true within the callback function

const nums = [9, 8, 7, 6, 5, 4, 3, 2, 1];
const odds = nums.filter((n) => {
  return n % 2 === 1; // our callback returns true or false
  // if it returns true, n is added to the filtered array
});
// [9, 7, 5, 3, 1]

const smallNums = nums.filter((n) => n < 5); // [4, 3, 2, 1]

We can also filter objects in an array:

const movies = [
  {
    title: 'Indiana Jones',
    score: 77,
  },
  {
    title: 'Star Trek',
    score: 94,
  },
  {
    title: 'Deadpool',
    score: 79,
  },
];

const badMovies = movies.filter((movie) => {
  return movie.score < 80;
});

console.log(badMovies);

Some and Every

• Some - tests if ANY of the elements in an array return true in the callback function. It returns a boolean

const firstWords = ['dog', 'jello', 'log', 'bag', 'wag', 'cupcake'];

words.some((word) => {
  return word.length > 4;
}); // true

words.some((word) => word[0] === 'Z'); // false
words.some((word) => word.includes('cake')); // true

• Every - tests if ALL the elements in an array return true in the callback function. It returns a boolean

const words = ['dog', 'dig', 'log', 'bag', 'wag'];

words.every((word) => {
  return word.length === 3;
}); // true

words.every((word) => word[0] === 'd'); // false

words.every((w) => {
  return w[w.length - 1] === 'g';
}); // true

Reduce

• The first (and potential only) argument in reduce is a callback function.
  • This function's first argument is a value that is subject to change as we iterate through each item.
  • The second argument in this function is the value from an index of the array.
• The optional second argument of reduce is the value that we want the callback function's first value to start at.

const prices = [9.99, 1.5, 19.99, 49.99, 30.5];

const total = prices.reduce((total, price) => {
  return total + price;
});

const min = prices.reduce((min, price) => {
  return Math.min(min, price);
});

console.log(min);

We can do this with an array of objects too:

const movies = [
  {
    title: 'Indiana Jones',
    score: 77,
  },
  {
    title: 'Star Trek',
    score: 94,
  },
  {
    title: 'Deadpool',
    score: 79,
  },
];

let bestMovie = movies.reduce((best, movie) => {
  if (movie.score > best.score) {
    return movie;
  }
  return best;
});

console.log(bestMovie);

We can also set the initial value of the reducer:

let nums = [1, 2, 3, 4, 5];

let totalPlus100 = nums.reduce((max, num) => {
  return (max += num);
}, 100);

console.log(totalPlus100); // 115