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
reduceis 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
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