svelte/compiler/validate/js/propValidators/methods.js

import checkForDupes from '../utils/checkForDupes.js';
import checkForComputedKeys from '../utils/checkForComputedKeys.js';

const builtin = {
	set: true,
	get: true,
	on: true,
	fire: true,
	observe: true,
	teardown: true
};

export default function methods ( validator, prop ) {
	if ( prop.value.type !== 'ObjectExpression' ) {
		validator.error( `The 'methods' property must be an object literal`, prop.start );
		return;
	}

	checkForDupes( validator, prop.value.properties );
	checkForComputedKeys( validator, prop.value.properties );

	prop.value.properties.forEach( prop => {
		if ( builtin[ prop.key.name ] ) {
			validator.error( `Cannot overwrite built-in method '${prop.key.name}'` );
		}
	});
}