Why do I need the v-bind when I have the v-on? - vue.js

Question

In the tutorial of vue.js, we have this code
<script>
export default {
data() {
return {
text: ''
}
},
methods: {
onInput(e) {
this.text = e.target.value
}
}
}
</script>
<template>
<input :value="text" #input="onInput" placeholder="Type here">
<p>{{ text }}</p>
</template>
And I don't understand why when I delete the bind on value, the two way binding is still working ?
In the tuto, it says that using the v-on & v-bind allow to do two way binding
Am I missing something ?

Answer 1

The Vue example is sort of a bad use case, a little simple for what it's trying to convey:
v-on is for assigning event listeners, so v-on:click="doSomething(value)"
v-bind is binding the actual value of vue data/state. So example:
<button v-on:click="setUserDetails(value)" v-bind:value="user.id">Click</button>

Answer 2

Imagine this component:
<template>
<input :value="value"/>
</template>
<script>
export default {
name: 'MyComp',
props:{
value: String
}
}
</script>
And now a simple usage of it:
<template>
<MyComp v-model="passwd" type="password" minlength="3" #focus="onFocus"/>
</template>
<script>
export default {
name: 'MyOtherComp',
data(){
return {
passwd: ''
}
},
methods:{
onFocus(){}
}
}
</script>
As you can see, value, type, and minlength properties and focus event are bidden to MyComp.
Now question: How can I handle extra props in MyComp? they are not defined in MyComp props. Vue gathers them in a special variable called $attrs, which is a normal JS object. Vue also gathers all events into $listeners variable.
Now inside MyComp these special variables are:
$atrrs:{
type: 'password',
minlength: '3'
}
$listerners:{
focus: /* function onFocus from parent */
}
To redirect these values:
<template>
<input :value="value" v-bind="$attrs" v-on="$listeners"/>
</template>
<script>
export default {
name: 'MyComp',
props:{
value: String
}
}
</script>
As you can see, we use v-bind to bind extra props, and we use v-on to bind (redirect) events. The result is:
<input :value="value" :type="$attrs.type" :minlength="$attrs.minlength" #focus="$listeners.focus"/>
Of course you can use these directions to bind you objects too:
<template>
<input :value="value" v-bind="$attrs" v-bind="accumulated" v-on="$listeners"/>
</template>
<script>
export default {
name: 'MyComp',
props:{
value: String
},
data(){
return {
accumulated:{
maxlenght: (+this.$attrs.minlength || 2) + 30, // It's just for a practice to use extra props inside JS code :-)
rows: 5,
}
}
}
}
</script>
Keep in mind that duplicate props will replace and the last one wins.