Svelte: getting to know Actions

Recently, an article from @sanReal appeared on Habr , where Alexander talked about what techniques and capabilities of Svelte he learned from his own experience. I was a little surprised not to see in his list the mention of one of the most powerful framework tools - Actions . In addition, communicating with people in the @sveltejs community who are already creating very good applications using Svelte, I sometimes notice that not everyone uses Actions, even where their application would ideally solve the problem. In this article I will tell you what Actions are and, with the simplest examples, I will show their application.

Suppose we have a text box in which the user must enter a phone number. We need to ensure that the user can enter only numbers in the field.

: +7 <input type="text" />

When the task is to endow a standard HTML element with additional features, it is common practice when working in any component of the framework to create a reusable component around this element, inside which the logic of non-standard behavior is implemented. You can do the same in Svelte:

<!-- InputDigits.svelte -->
<script>
    function clean_value(e){
        e.target.value =  e.target.value.replace(/[^\d]/g,'');
    }
</script>

<input type="text" on:input={clean_value} />

<!-- App.svelte -->
...
: +7 <InputDigits />

View in action

The task has been completed, but the element itself is <input/>now hidden inside the component, so we will no longer be able to manipulate it as a regular HTML element - to "hang" on it any CSS classes, event handlers, or some special attributes will not be so simple.

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:

<script>
    function onlydigits(node) {

        function clean_value(){
            node.value = node.value.replace(/[^\d]/g,'');
        }

        node.addEventListener('input',clean_value);

        return {
            destroy: ()=>node.removeEventListener('input',clean_value)
        }

    }
</script>

: +7 <input type="text" use:onlydigits />

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<script>
    function onlydigits(node) { ... }

    function max10symbols(node) {

        function trim_value(){
            node.value = node.value.substring(0, 10);
        }

        node.addEventListener('input',trim_value);

        return {
            destroy: ()=>node.removeEventListener('input',trim_value)
        }

    }
</script>

: +7 <input type="text" use:onlydigits use:max10symbols />

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<script>
    function format_by_pattern(node,pattern) {

        function set_cursor(){
            const match = node.value.match(/[\d]/gi);
            const pos = match ? node.value.lastIndexOf(match.pop())+1 : 0;
            node.setSelectionRange(pos, pos);
        }

        function format_value(e){
            let digits = node.value.replace(/[^\d]/g,'').split('');
            node.value = pattern.replace(/[*]/g,(m)=>digits.shift()||m);
            set_cursor();
        }

        node.addEventListener('input',format_value);

        format_value();

        return {
            destroy: ()=>node.removeEventListener('input',format_value)
        }

    }
</script>

: +7 <input type="text" use:format_by_pattern={'(***) ***-**-**'} /> <br/>
: <input type="text" use:format_by_pattern={' **** №******'} />

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<script>
    function format_by_pattern(node,pattern) {

        function set_cursor(){...}
        function format_value(){...}

        ...

        return {
            destroy: ...,
            update: (new_pattern)=>{
                pattern=new_pattern;
                format_value();
            }
        }

    }

    let zagran = false;
</script>

: +7 <input type="text" use:format_by_pattern={'(***) ***-**-**'} /> <br/>
: <input type="text" use:format_by_pattern={zagran ? ' ** №*******' : ' **** №******'} /> <br/>
<input type="checkbox" bind:checked={zagran}/> - 

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Note: Very soon in Moscow will be held Svelte Russia Meetup # 1 . Register, there is still room. For those who will not be able to attend in person, a broadcast and recording of the event will be organized.