prop did't produce right value

[yangjunjun]

Version

2.5.17

Reproduction link

https://jsfiddle.net/yangjunjun/eywraw8t/450799/

Steps to reproduce

see the demo, it's simple

What is expected?

the first prop 'val' should be empty string

What is actually happening?

the first prop 'val' is true

---

when I create a checkbox-like component which has a props "value" who accept Boolean String type. when an empty string was assign to the "value" but the actual value is "true", it's weird.

[Justineo]

As you declared the prop as type [Boolean, String], Vue would try to figure out it can be treated as a boolean value first. And for boolean props, as long as it exists it's regarded as truthy.

<!-- the following declarations are equivalent -->
<input disabled>
<input disabled="">
<input disabled="whatever">

You can declare the type as [String, Boolean] if you want to regard the value as string first.

[yangjunjun]

It doesn't make sense. the order of props type should not affect the result. the props accept String and I assign string, it should be a string. Now it's confusing.
Anyway the official docs didn't mention this.

[Justineo]

And this implementation (depending on the order of the type array) is intentional:

vue/src/core/util/props.js

Lines 36 to 41 in d483a49

	// only cast empty string / same name to boolean if
	// boolean has higher priority
	const stringIndex = getTypeIndex(String, prop.type)
	if (stringIndex < 0 || booleanIndex < stringIndex) {
	value = true
	}

If you feel this is still confusing, welcome to file an issue or make a pull request to our documentation repo: vuejs/vuejs.org. Thanks.

[KaelWD]

See #7485 and the linked issues