Elyses Analytic Enchantments

Arrow Functions

Besides function declarations and function expressions, JavaScript also has another very concise syntax for defining a function. These functions are called arrow functions.

Here is a comparison between a function declaration and an arrow function.

function addUpTwoNumbers(num1, num2) {
  return num1 + num2;
}
// function keyword removed and => added
const addUpTwoNumbers = (num1, num2) => {
  return num1 + num2;
};

If the function body contains only a return statement, like in the example above, the {} and the return keyword can be omitted. If there is only one parameter, the parenthesis () can be omitted as well.

const addUpTwoNumbers = (num1, num2) => num1 + num2;
const square = num => num * num;

Arrow functions are often used to define short callback functions directly in the function call.

applyToSquare(number => number * number);

Array Analysis

Arrays have built-in methods to analyse the contents of the array. Most of these methods take a function that returns true or false as an argument. Such a function is called a predicate.

The built-in methods are meant to be used instead of a for loop or the built-in forEach method:

Example of analysis using a for loop :

const numbers = [1, 'two', 3, 'four'];
for (var i = 0; i < numbers.length; i++) {
  if (numbers[i] === 'two') {
    return i;
  }
}
// => 1

Example of analysis using a built-in method:

const numbers = [1, 'two', 3, 'four'];
numbers.indexOf('two');
// => 1

Some other helpful built-in methods that are available to analyze an array are shown below. See MDN for a full list of array methods.

includes

The includes() method determines whether an array includes a certain value among its entries, returning true or false as appropriate. ¹

const numbers = [1, 'two', 3, 'four'];
numbers.includes(1);
// => true
numbers.includes('one');
// => false

every

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value. ²

const numbers = [1, 3, 5, 7, 9];
numbers.every((num) => num % 2 !== 0);
// => true

some

The some() method tests whether at least one element in the array passes the test implemented by the provided function. ³

const numbers = [1, 3, 5, 7, 9];
numbers.some((num) => num % 2 !== 0);
// => true

find

The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned. ⁴

const numbers = [1, 3, 5, 7, 9];
numbers.find((num) => num < 5);
// => 1

findIndex

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test. ⁵

const numbers = [1, 3, 5, 7, 9];
numbers.findIndex((num) => num > 7);
// => 4
numbers.findIndex((num) => num > 9);
// => -1

1. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes ↩

2. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every ↩

3. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some ↩

4. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find ↩

5. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex ↩