🔰event handler

browser ⟩ event ⟩ handler

event (function) handler

• argument：an Event object.

• invocation context：

  • this refers to the object on which the handler was registered.

• return value： In modern JavaScript, event handlers should not return anything.

event handler can also be an object - 👉 see: object handler

🧨 雷區

Window-reflecting body element event handlers：

if you assign one of the following event handlers on document.body, it will be reflected on window too, and vice versa.

• onblur, onerror, onfocus, onload, onresize, onscroll

const body = document.body;
body.onblur = function f(){};        // assigned on body
window.onblur                        // f: reflected on window

window.onresize = function g(){};    // assigned on window
body.onresize                        // g: reflected on body

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