for ( let i = 0; i < 10; i += 1 ) console.log( i );
for ( const letter of array ) console.log( letter );
for ( const index in array ) console.log( index );

let i;
for ( i = 0; i < 10; i += 1 ) console.log( i );

let letter;
for ( letter of array ) console.log( letter );

let index;
for ( index in array ) console.log( index );