Vue.js Interview Questions

Vue cannot detect changes for the object in property addition or deletion.

Lets take an example of user data changes,

var vm = new Vue({
data: {
user: {
name: 'John'
}
}
})

// `vm.user.name` is now reactive

vm.user.email = john@email.com // `vm.user.email` is NOT reactive

You can overcome this scenario using the Vue.set(object, key, value) method or Object.assign(),

Vue.set(vm.user, 'email', 'john@email.com');
// (or)
vm.user = Object.assign({}, vm.user, {
email: john@email.com
})

You can also use integer type(say 'n') for v-for directive which repeats the element many times.

<span v-for="n in 20">{{ n }} </span>

It displays the number 1 to 20.

Just similar to v-if directive on template, you can also use a <template> tag with v-for directive to render a block of multiple elements.

Let's take a todo example,

<ul>
<template v-for="todo in todos">
<li>{{ todo.title }}</li>
<li class="divider"></li>
</template>
</ul>

You can use event handlers in vue similar to plain javascript. The method calls also support the special $event variable.

<button v-on:click="show('Welcome to VueJS world', $event)">
Submit
</button>

methods: {
show: function (message, event) {
// now we have access to the native event
if (event) event.preventDefault()
console.log(message);
}
}

Normally, javascript provides event.preventDefault() or event.stopPropagation() inside event handlers. You can use methods provided by vue, but these methods are meant for data logic instead of dealing with DOM events. Vue provides below event modifiers for v-on and these modifiers are directive postfixes denoted by a dot.

1. .stop
2. .prevent
3. .capture
4. .self
5. .once
6. .passive

Let's take an example of stop modifier,

<!-- the click event's propagation will be stopped -->

You can also chain modifiers as below,

<!-- modifiers can be chained -->

Vue supports key modifiers on v-on for handling keyboard events. Let's take an example of keyup event with enter keycode.

<!-- only call `vm.show()` when the `keyCode` is 13 -->
<input v-on:keyup.13="show">

Remembering all the key codes is really difficult. It supports the full list of key codes aliases

1. .enter
2. .tab
3. .delete (captures both "Delete" and "Backspace" keys)
4. .esc
5. .space
6. .up
7. .down
8. .left
9. .right

Now the above keyup code snippet can be written with aliases as follows,

<input v-on:keyup.enter="submit" />
<!-- OR with shorthand notation -->
<input @keyup.enter="submit" />

Note: The use of keyCode events is deprecated and may not be supported in new browsers.

You can define custom key modifier aliases via the global config.keyCodes. There are few guidelines for the properties

1. You can't use camelCase. Instead you can use kebab-case with double quotation marks
2. You can define multiple values in an array format

Vue.config.keyCodes = {
f1: 112,
"media-play-pause": 179,
down: [40, 87]
}

Vue supports below modifiers to trigger mouse or keyboard event listeners when the corresponding key is pressed,

1. .ctrl
2. .alt
3. .shift
4. .meta

Lets take an example of control modifier with click event,

<!-- Ctrl + Click -->

Vue supports below mouse button modifiers

1. .left
2. .right
3. .middle

For example, the usage of .right modifier as below

<button
v-if="button === 'right'"
v-on:mousedown.right="increment"
v-on:mousedown.left="decrement"
/>

You can use the v-model directive to create two-way data bindings on form input, textarea, and select elements.

Lets take an example of it using input component,

<input v-model="message" placeholder="Enter input here">

Remember, v-model will ignore the initial value, checked or selected attributes found on any form elements. So it always use the Vue instance data as the source of truth.

There are three modifiers supported for v-model directive.

1. lazy: By default, v-model syncs the input with the data after each input event. You can add the lazy modifier to instead sync after change events.

<!-- synced after "change" instead of "input" -->
<input v-model.lazy="msg" >

2. number: If you want user input to be automatically typecast as a number, you can add the number modifier to your v-model. Even with type="number", the value of HTML input elements always returns a string. So, this typecast modifier is required.

<input v-model.number="age" type="number">

3. trim: If you want whitespace from user input to be trimmed automatically, you can add the trim modifier to your v-model.

<input v-model.trim="msg">

Components are reusable Vue instances with a name. They accept the same options as new Vue, such as data, computed, watch, methods, and lifecycle hooks(except few root-specific options like el).

Lets take an example of counter component,

// Define a new component called button-counter
Vue.component('button-counter', {
template: '<button v-on:click="count++">You clicked me {{ count }} times.</button>'
data: function () {
return {
count: 0
}
},
})

Let's use this component inside a root Vue instance created with new Vue

<button-counter></button-counter>

var vm = new Vue({ el: '#app' });

Props are custom attributes you can register on a component. When a value is passed to a prop attribute, it becomes a property on that component instance. You can pass those list of values as props option and use them as similar to data variables in template.

Vue.component('todo-item', {
props: ['title'],
template: '<h2>{{ title }}</h2>'
})

Once the props are registered, you can pass them as custom attributes.

<todo-item title="Learn Vue conceptsnfirst"></todo-item>

In VueJS 2.x, every component must have a single root element when template has more than one element. In this case, you need to wrap the elements with a parent element.

<template>
<h2>{{ title }}</h2>
</template>

Otherwise there will an error throwing, saying that "Component template should contain exactly one root element...".

Whereas in 3.x, components now can have multiple root nodes. This way of adding multiple root nodes is called as fragments.

<template>
<h2>{{ title }}</h2>
</template>

If you want child wants to communicate back up to the parent, then emit an event from child using $emit object to parent,

Vue.component('todo-item', {
props: ['todo'],
template: `
<h3>{{ todo.title }}</h3>
<button v-on:click="$emit('increment-count', 1)">
Add
</button>
`
})

Now you can use this todo-item in parent component to access the count value.

<ul v-for="todo in todos">
<li>
<todo-item
v-bind:key="todo.id"
v-bind:todo="todo"
v-on:increment-count="total += 1"
/></todo-item>
</li>
</ul>
<span> Total todos count is {{total}}</span>