VueJS conditionally add an attribute for an element

166
votes

In VueJS we can add or remove a DOM element using v-if:

<button v-if="isRequired">Important Button</button>

but is there a way to add / remove attributes of a dom element eg for the following conditionally set the required attribute:

Username: <input type="text" name="username" required>

by something similar to:

Username: <input type="text" name="username" v-if="name.required" required>

Any ideas?

8
javascriptvue.jsvuejs2
While being not so obvious (hence the confusion) the documentation actually do says that if attribute value evaluates to false then attribute being omitted (vuejs.org/v2/guide/syntax.html#Attributes) – AlexanderB
Actually, the documentation says the attribute won't be added if “…has the value of null, undefined, or false”, which is different from a JS script evaluating to false. This means an empty string is falsy in JavaScript, but would still add the attribute to DOM. To prevent that you could try v-bind:name="name || false" – Denilson Sá Maia
@AlexanderB If that's true, how can I pass explicit false to child component via a prop? – Bruce Sun
@BruceSun , If the attribute in context "unintentionally" disappears when you give it false value - try to pass it as a string 'false'. In other cases when you need to control presence of non-boolean html attribute on the element you can use conditional rendering with v-if as suggested here: github.com/vuejs/vue/issues/7552#issuecomment-361395234 – AlexanderB
@AlexanderB I think I have to correct myself - I should say attribute but NOT prop. We can safely pass explicit false via a component property but NOT attribute (which is not recognised as a property). Am I correct? – Bruce Sun

8 Answers

167
votes

Try:

<input :required="test ? true : false">

Update: It has changed in Vue 3, see this answer https://stackoverflow.com/a/64598898

75
votes

Simplest form:

<input :required="test">   // if true
<input :required="!test">  // if false
<input :required="!!test"> // test ? true : false
20
votes

<input :required="condition">

You don't need <input :required="test ? true : false"> because if test is truthy you'll already get the required attribute, and if test is falsy you won't get the attribute. The true : false part is redundant, much like this...

if (condition) {
    return true;
} else {
    return false;
}
// or this...
return condition ? true : false;
// can *always* be replaced by...
return (condition); // parentheses generally not needed

The simplest way of doing this binding, then, is <input :required="condition">

Only if the test (or condition) can be misinterpreted would you need to do something else; in that case Syed's use of !! does the trick.
  <input :required="!!condition">

18
votes

You can pass boolean by coercing it, put !! before the variable.

let isRequired = '' || null || undefined
<input :required="!!isRequired"> // it will coerce value to respective boolean

But I would like to pull your attention for the following case where the receiving component has defined type for props. In that case, if isRequired has defined type to be string then passing boolean make it type check fails and you will get Vue warning. To fix that you may want to avoid passing that prop, so just put undefined fallback and the prop will not sent to component

let isValue = false
<any-component
  :my-prop="isValue ? 'Hey I am when the value exist' : undefined"
/>

Explanation

I have been through the same problem, and tried above solutions !! Yes, I don't see the prop but that actually does not fulfils what required here.

My problem -

let isValid = false
<any-component
  :my-prop="isValue ? 'Hey I am when the value exist': false"
/>

In the above case, what I expected is not having my-prop get passed to the child component - <any-conponent/> I don't see the prop in DOM but In my <any-component/> component, an error pops out of prop type check failure. As in the child component, I am expecting my-prop to be a String but it is boolean.

myProp : {
 type: String,
 required: false,
 default: ''
}

Which means that child component did receive the prop even if it is false. Tweak here is to let the child component to take the default-value and also skip the check. Passed undefined works though!

<any-component
  :my-prop="isValue ? 'Hey I am when the value exist' : undefined"
/>

This works and my child prop is having the default value.

17
votes

Conditional rendering of attributes changed in Vue 3. To omit an attribute use null or undefined.

Vue 2:

<div :attr="false">
Result: <div>

<div :attr="null">
Result: <div>

Vue 3:

<div :attr="false">
Result: <div attr="false">

<div :attr="null">
Result: <div>
8
votes

It's notable to understand that if you'd like to conditionally add attributes you can also add a dynamic declaration:

<input v-bind="attrs" />

where attrs is declared as an object:

data() {
    return {
        attrs: {
            required: true,
            type: "text"
        }
    }
}

Which will result in:

<input required type="text"/>

Ideal in cases with multiple attributes.

8
votes

You can add colon before attribute (also can use conditions) like

<div :class="current? 'active': '' " > 
<button :disabled="InvalidForm? true : false " >

If you want to set a dynamic value like props then you also can use colon before attribute name like :

<Child :data="userList" />
2
votes

In html use

<input :required="condition" />

And define in data property like

data () {
   return {
      condition: false
   }
}