All objects in JS are truthy? except him...

Hi everyone, I'm Kasong.

According to the ES specification , all non-primitive objects converted to Boolean are all true :

But here is a special case: document.all

This article will talk about this cold knowledge.

The role of document.all

document.all will return a HTMLAllCollection collection, including all nodes under document It can be understood as a containing all DOM nodes in the page :

Early web developers used the API obtain the DOM node, such as:

// 获取页面中第一个节点，即HTML
document.all[0];

// 获取页面中id为"abcd"的节点
document.all["abcd"];

With the WEB , its role has been gradually replaced by other methods of document

• getElementById
• querySelector

......

W3C forward with weight

Although there is a better API , but many old pages are still using document.all . How to make a better transition?

For compatibility, many developers will write the following code:

if (document.all) {
  // 老浏览器
} else if (document.getElementById) {
  // 支持getElementById的现代浏览器
}

The problem is that many modern browsers also implement document.all , which allows the code to enter the old in all browsers.

From June to October 2009, there were as many as 56 messages on the W3C mailing list document.all

The final result of the discussion is: In modern browsers, document.all transformed into Boolean the result is false .

In this way, the above code will not enter the logic of old browser

Summarize

In addition to the above characteristics, document.all also has some characteristics different from ordinary people, such as:

• When used as == and != operators, it will be treated as undefined
• typeof document.all === 'undefined'

These weird results are for browser backward compatibility. Although many modern browsers support document.all , it has been removed from the standard.

I can think of his only role at the moment, which is probably to challenge the interviewer...

Welcome to join human high-quality front-end framework group , take flight