findIndex

If the array item is an object, you can use the findIndex method

const letters = [ 
  { 
    letter: 'A',  
  }, 
  { 
    letter: 'B',  
  }, 
  { 
    letter: 'C',  
  }, 
] 

const index = letters.findIndex((element, index) => { 
  if (element.letter === 'B') {  
    return true 
  } 
}) 
console.log(index);   // 1