JavaScript - Handling events - Asynchronous Coding - Events and queues(mouse move event(onmousemove), pageX, pageY)

Head First JavaScript Programming

開発環境

• macOS Sierra - Apple (OS)
• Emacs (Text Editor)
• JavaScript (プログラミング言語)
• Safari(Web browser)

Head First JavaScript Programming (Eric T. Freeman (著)、Elisabeth Robson (著)、O'Reilly Media)の Chapter 9.(Handling events - Asynchronous Coding)の EXERCISE(No. 6015)を取り組んでみる。

EXERCISE(No. 6015)

コード(Emacs)

HTML5

<pre id="output0"></pre>
<button id="run0">run</button>
<button id="clear0">clear</button>
<br>
<img id="map" width="600" height="200" src="">
<p id="coords">Move mouse to find coordinates…</p>
<script src="sample2.js"></script>

JavaScript

let btn0 = document.querySelector('#run0'),
    btn1 = document.querySelector('#clear0'),
    pre0 = document.querySelector('#output0'),
    p = (x) => pre0.textContent += x + '\n';

let b0 = document.querySelector('body');

let init = () => {
    let map = document.querySelector('#map');

    map.onmousemove = showCoords;
};

let showCoords = (event) => {
    let coords = document.querySelector('#coords');

    let x = event.pageX,
        y = event.pageY;
    
    coords.textContent = `Map coordinates: ${x}, ${y}`;
};

let output = () => {
    init();
};

let clear = () => {};
btn0.onclick = output;
btn1.onclick = clear;

output();

Move mouse to find coordinates…