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Handling Form Input with Vue.js — Input Bindings

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Vue.js is an easy to use web app framework that we can use to develop interactive front end apps.

In this article, we’ll look at how to bind data in various ways and automatically modify inputs.

Value Bindings

v-model bind values usually as static strings, or booleans for a checkbox.

For example, for checkbox, we can bind it to a string as follows:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    selected: undefined  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      foo  
      <input type="radio" v-model="selected" value="foo" />  
      <br />  
      bar  
      <input type="radio" v-model="selected" value="bar" />  
      <br />  
      <span>Selected: {{ selected }}</span>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

Checkboxes are bound to a boolean value:

<input type="checkbox" v-model="toggle">

Select are bound to the selected string set in the value attribute:

<select v-model="selected">     
  <option value="foo">Foo</option>   
</select>

Checkbox

We can change the true and false value of a checkbox with the true-value and false-value attributes as follows:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    selected: undefined  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      Toggle  
      <input  
        type="checkbox"  
        v-model="selected"  
        true-value="yes"  
        false-value="no"  
      />  
      <br />  
      <span>Selected: {{ selected }}</span>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

Then we get yes when we checked the checkbox and no otherwise.

true-value and false-value don’t affect the input’s value attribute because browsers don’t include unchecked boxes in form submissions.

We should use radio inputs to guarantee at least one item is selected.

Radio Buttons

We can bind the value attribute to the selected value with v-bind:value .

For example, we can write the following:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    pick: "",  
    foo: "foo",  
    bar: "bar"  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      Foo  
      <input type="radio" v-model="pick" v-bind:value="foo" />  
      <br />  
      Bar  
      <input type="radio" v-model="pick" v-bind:value="bar" />  
      <br />  
      <p>Selected: {{ pick }}</p>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

Then v-bind:value=”foo” renders as value='foo' and v-bind:value=”bar” is rendered as value='bar' .

When we pick a value, we get foo or bar depending on which radio button is clicked.

Select Options

We can bind a value to an object as follows:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    selected: undefined  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      <select v-model="selected">  
        <option v-bind:value="{ number: 123 }">123</option>  
        <option v-bind:value="{ number: 456 }">456</option>  
      </select>  
      <br />  
      <p>Selected: {{ selected }}</p>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

Then we get either { number: 123 } or { number: 456 } displayed depending on what’s selected.

If we want to display the value of the number property, then we have to write the following:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    selected: {}  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      <select v-model="selected">  
        <option v-bind:value="{ number: 123 }">123</option>  
        <option v-bind:value="{ number: 456 }">456</option>  
      </select>  
      <br />  
      <p>Selected: {{ selected.number }}</p>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

Since setting the initial value to undefined will give us an error and the app won’t run.

Modifiers

Vue comes with a few modifiers for v-model .

.lazy

We can use v-model.lazy to sync data on change events rather than input events. This means that changes are only synced when the input it out of focus.

For example, we can use it as follows:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    msg: ""  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      <input v-model.lazy="msg" />  
      <br />  
      <p>Value: {{ msg }}</p>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

When we move out cursor out of the input, then we get the input value displayed.

.number

The number modifier will convert whatever’s entered into a number. This is useful because input with type='number' still returns a string.

For example, we can use it as follows:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    age: 0  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      <input v-model.number="age" type="number" />  
      <br />  
      <p>Age: {{ age }}</p>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

.trim

We can trim whitespace automatically with the .trim modifier. To use it, we can write the following:

src/index.js :

new Vue({  
  el: "#app",  
  data: {  
    msg: ""  
  }  
});

index.html :

<!DOCTYPE html>  
<html>  
  <head>  
    <title>App</title>  
    <meta charset="UTF-8" />  
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>  
  </head> <body>  
    <div id="app">  
      <input v-model.trim="msg" type="text" />  
      <br />  
      <p>Value: "{{ msg }}"</p>  
    </div> <script src="src/index.js"></script>  
  </body>  
</html>

Then when we type in something with starting and ending whitespaces, they won’t show up.

So:

" a b c "

will become "a b c" .

Conclusion

We can bind dynamically to values with the v-bind:value attribute. It lets us bind value attributes to primitive values and objects.

Also, we can use various modifiers on v-model to automatically change the input value in various ways like trimming input and converting them to numbers.

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