can i pass function as attribute to web component?

Question

I'm trying to create a native web component for input element. I wanted the component to have custom validation functionality, similar to polymer's paper-input custom validator function. I'm not sure if I can pass a custom validator function as attribute to an instance of (web component) input element. Any suggestions would be appreciated.

Answer 1

An attribute is a string, not a function. You can pass a a function as a string and then evaluate it with the eval() function. It's not considered as a good practice, for security reasons.

Another solution is to pass it to the custom element as a Javascript property:

function validate( value ) { return Number.isInteger( value) }
myCustomElement.validation = validate

Or, using a arrow function:

myCustomElement.validation = v => Number.isInteger( va )

class CustomInput extends HTMLElement {
    constructor() {
        super()
        var sh = this.attachShadow( { mode: 'open' } )
        sh.appendChild( tpl.content.cloneNode( true ) )
        
        var div = sh.querySelector( 'div' )
        div.addEventListener( 'input', () => { 
           if ( !this.validate( Number( div.textContent ) ) )
            div.classList.add( 'error' )
           else
            div.classList.remove( 'error' ) 
        } )        
    }
}

customElements.define( 'custom-input', CustomInput )

integer.validate = v => Number.isInteger( v )

<template id="tpl">
  <style>
    :host {
       display: inline-block ;
       min-width: 150px ;
       border: 1px solid cyan ;
    }
    
    div.error {
       color: red ;
    }

  </style>
  <div contenteditable></div>
</template>
    
<custom-input id="integer"></custom-input>