General Promise Usage

general-promise-usage.js

'use strict';

const students = [
    {
        id: 1,
        name: 'Bart',
    },
    {
        id: 2,
        name: 'Lisa',
    },
    {
        id: 3,
        name: 'Gisborne',
    },
];

const grades = [
    {
        studentId: 1,
        scores: [55, 93, 23],
    },
    {
        studentId: 2,
        scores: [45, 67, 98],
    },
    {
        studentId: 3,
        scores: [32, 76, 28],
    },
];

let getStudentId = (studentName) => {
    return new Promise((resolve, reject) => {
        let result = undefined;

        for (let student of students) {
            if (student.name === studentName) {
                result = student.id;
                break;
            }
        }

        if (result === undefined) {
            reject(null);
        } else {
            resolve(result);
        }
    });
};

let getStudentGrades = (studentId) => {
    return new Promise((resolve, reject) => {
        let result = null;

        for (let grade of grades) {
            if (grade.studentId === studentId) {
                result = grade.scores;
                break;
            }
        }

        if (result === null) {
            reject(null);
        } else {
            resolve(result);
        }
    });
};

let studentName = 'Lisa';
getStudentId(studentName).then((studentId) => {
    return getStudentGrades(studentId);
}).then((scores) => {
    let sum = 0;

    for (let score of scores)
        sum += score;

    let average = sum/3;

    console.log(`${studentName}'s result is ${average} %.`);
}).catch((error) => {
    console.log(`${studentName} not found!`);
});