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(value) method determines whether an array includes a certain value.
It returns true when the value is included, false otherwise.
const numbers = [1, 'two', 3, 'four'];
numbers.includes(1);
// => true
numbers.includes('one');
// => false
every
The every(predicate) method take a function which is a predicate.
It tests whether all elements in the array return true when passed to the predicate.
In other words: the methods tests that all its elements pass the test passed to the function call.
It returns true when every element passes the predicate test, false otherwise.
const numbers = [1, 3, 5, 7, 9];
numbers.every((num) => num % 2 !== 0);
// => true
some
The some(predicate) method is the same as the every method, but returns true if at least one item in the array passes the predicate test.
const numbers = [1, 3, 5, 7, 9];
numbers.some((num) => num % 2 !== 0);
// => true
find
The find(predicate) method returns the value of the first element in the array that passes the predicate test.
Where some() returns true when it passes, find() returns the actual value in the array.
The method returns undefined when none of the elements in the array pass the predicate test.
const numbers = [1, 3, 5, 7, 9];
numbers.find((num) => num < 5);
// => 1
findIndex
The findIndex(predicate) is the same as the find() method, but it returns the (first) index of the element that passes the predicate test instead of the value.
The method returns -1 when none of the elements in the array pass the predicate test.
const numbers = [1, 3, 5, 7, 9];
numbers.findIndex((num) => num > 7);
// => 4
numbers.findIndex((num) => num > 9);
// => -1
Learning Exercise
