VueJS conditionally add an attribute for an element

Try:

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

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

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>

Simplest form:

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

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.

<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">

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" />

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.

You could write something like this:

<input type="text" name="username" :required="condition ? true : false">

In html use

<input :required="condition" />

And define in data property like

data () {
   return {
      condition: false
   }
}

You can use computed too

<input :required="isRequired" />

Computed:

computed: {
   isRequired () {
      return someLogic //Boolean, true or false
   }

use v-bind

v-bind="{ ...(type && { variant: type })

In vuejs you have something called v-show which conditionally render the element within the DOM, whereas v-if just won't show on the screen but would still exist on DOM.

You can try this code

use template, don't use the v-show condition within the button attributes

<template v-show="isRequired">
    <button >Important Button</button>
</template>

this "isRequired" seems like its a computed property so make sure you specify the correct logic within that

I had a form that would require to fill bank detail input filled; if user selected that it had an account.

My solution (using Vue 3 composition) is simple

Create a ref variable initially set to false; that would be used to either have the required attribute or not

const isRequired = ref(false)

I added on change event on account;

<select id="account" v-model="form.account" 
@change="isRequired=!isRequired">
        <option value="YES">YES</option>
        <option value="NO">NO</option>
      </select>

<input type="text" name="bank_details" v-model="form.bank_details" 
:required="isRequired" />

so when user select that they have an account; bank-detail input would have the required attribute added to it.

From typescript:

type 'string | undefined'

If you not need attribute set value to undefined