JS: Array.prototype.findIndex

New in JS2015.

• findIndex return the index of the element.
• find return the element.

See Array.prototype.find

const isEven = (x => ( x % 2 === 0 ? true : false ) )

console.log( [7,3,4,2].findIndex (isEven) ); // 2

// if not found, return undefined
console.log( [1,3,5].find (isEven) ); // undefined