[JavaScript] 9. Array API

 

1. join

 - make a string out of an array.

 - join(separator?: string): string;

'use strict';

const fruits = ['apple', 'banana', 'orange'];
const result = fruits.join(',');
console.log(result); // apple, banana, orange

 

 

2. split

 - make an array out of a string.

 - split(separator: string | RegExp, limit?: number): string[];

const fruits = 'apple, banana, orange';
const result1 = fruits.split(',');
console.log(result1); // ["apple", " banana", " orange"]
const result1 = fruits.split(',', 2);
console.log(result2); // ["apple", " banana"]

 

 

3. reverse

 - make this array look like this: [5, 4, 3, 2, 1]

const arr = [1, 2, 3, 4, 5];
const result = arr.reverse();
console.log(result); // [5, 4, 3, 2, 1]
console.log(arr); // [5, 4, 3, 2, 1]

 

 

4. slice

 - make new array without the first two elements.

 - slice(start?: number, end?: number): T[];

const arr = [1, 2, 3, 4, 5];
const result = arr.slice(2, 5);
console.log(result); // [3, 4, 5]
console.log(arr); // [1, 2, 3, 4, 5]

 

 

---

class Student {
	constructor(name, age, enrolled, score) {
    	this.name = name;
        this.age = age;
        this.enrolled = enrolled;
        this.score = score;
    }
}

const students = [
	new Student('A', 29, true, 45),
    new Student('B', 28, false, 80),
    new Student('C', 30, true, 90),
    new Student('D', 40, false, 66),
    new Student('E', 18, true, 88),
];

 

5. find

 - Returns the value of the first element in the array where predicate is true, and undefined.

// find a student with the score 90.
const result = students.find(function(student, index) {
	return student.score === 90;
}); // students.find((student) => student.score === 90);로 생략 가능

console.log(result); // Student {name: "C", age: 30, enrolled: true, score: 90}

 

 

6. filter

 - Returns the elements of an array that meet the condition specified in a callback function.

const result = students.filter((student) => student.enrolled);
console.log(result);

 

 

7. map

 - Calls a defined callback function on each element of an array.

const result = students.map((student) => student.score*2);
console.log(result); // [90, 160, 180, 132, 176]

 

 

8. some & every

 - Determines whether the specified callback function returns true.

// check if there is a student with the score lower than 50.
const result = students.some((student) => student.score < 50);
console.log(result); // true

result = !students.every((student) => student.score >= 50);
console.log(result); // true

 

 

9. reduce

 - Calls the specified callback function for all the elements in an array.

const result = students.reduce((prev, curr) => prev + curr.score, 0);
console.log(result / students.length); // 73.8

 

 

---

Quiz1. make a string containing all the scores bigger than 50.

 - result should be: '80,90,66,88'

const result = students
	.map((student) => student.score)
    .filter((score) => score >= 50)
    .join();
console.log(result); // 80,90,66,88

 

 

Quiz2. do sorted in ascending order.

 - result should be: '45,66,80,88,90'

const result = students
	.map(student => student.score)
    .sort((a, b) => a - b)
    .join();
console.log(result); // 45,80,90,66,88