javascript - setgetAttribute comparisons only work with strings? - Stack Overflow

admin管理员组

文章数量:1324231

I would like to set a span node's visited attribute to true or false based on if it has been visited.

test();

function test () {

    var el = document.createElement("span");
    el.setAttribute("visited", false);

    el.setAttribute("visited", true);

    alert(el.getAttribute("visited") === true); //False
    alert(el.getAttribute("visited") === "true"); //True

}

I initially set the attribute "visited" to boolean false, then set the boolean to true. I noticed that when I checked if the attribute was true, it returned false, but if I checked the string true, it returned true.

The MSN Docs only talk about the attributeName as needing to be string, not the value. So why doesn't paring against bools work?

FIDDLE

I would like to set a span node's visited attribute to true or false based on if it has been visited.

test();

function test () {

    var el = document.createElement("span");
    el.setAttribute("visited", false);

    el.setAttribute("visited", true);

    alert(el.getAttribute("visited") === true); //False
    alert(el.getAttribute("visited") === "true"); //True

}

I initially set the attribute "visited" to boolean false, then set the boolean to true. I noticed that when I checked if the attribute was true, it returned false, but if I checked the string true, it returned true.

The MSN Docs only talk about the attributeName as needing to be string, not the value. So why doesn't paring against bools work?

FIDDLE

• javascript
• html
• boolean
• setattribute
• getattribute

Share Follow edited Mar 4, 2015 at 20:32 sfletche 49.8k3131 gold badges108108 silver badges120120 bronze badges asked Mar 4, 2015 at 20:03 user3871user3871 12.7k3636 gold badges140140 silver badges282282 bronze badges 2

• 7 Yes, attributes are always strings, but properties can be several different types. – KJ Price Commented Mar 4, 2015 at 20:04
• MSDN isn't a normative reference for the relevant standard, the W3C is: setAttribute, getAttribute. – RobG Commented Mar 4, 2015 at 20:37

Add a ment  | 

3 Answers 3

Sorted by: Reset to default 8

This is because getAttribute return type is string not bool

Return Value: A String, representing the specified attribute's value.

Note: If the attribute does not exist, the return value is null or an empty string ("")

The === operator checks both value and type (with no implicit coercion of types).

Since getAttribute returns a string value, the parison is only true when pared to the string "true" and not when pared to the boolean value of true.

To put it another way, when using the === operator...

true === 'true' // false
'true' === 'true' // true
true === true // true

Here is the definition of attributes as defined by the HTML standard:

3.2.3.1 Attributes

Except where otherwise specified, attributes on HTML elements may have any string value, including the empty string. Except where explicitly stated, there is no restriction on what text can be specified in such attributes.

So, to repeat what was already stated, HTML attributes are always strings.

本文标签： javascriptsetgetAttribute comparisons only work with stringsStack Overflow