Javascript Map Array Last Item

Try something like:

row.map((rank, i, row) => {
  if (i + 1 === row.length) {
    // Last one.
  } else {
    // Not last one.
  }
})

Old answer:

const rowLen = row.length;
row.map((rank, i) => {
  if (rowLen === i + 1) {
    // last one
  } else {
    // not last one
  }
})

As LeoYuan answered, this is the correct answer, but it can be a bit improved.
map accepts a function with a third parameter, which is the iterated array itself.

row.map((rank, i, arr) => {
    if (arr.length - 1 === i) {
        // last one
    } else {
        // not last one
    }
});

or in a bit shorter version, using an object destructuring (thanks Jose from the comments):

row.map((rank, i, {length}) => {
    if (length - 1 === i) {
        // last one
    } else {
        // not last one
    }
});

Using an arr.length instead of row.length is a better and correct approach for several reasons:

1. When you mix scopes, it may lead for an unexpected bugs, especially in a poorly written or poorly designed code. In general, it is always a good way to avoid mixing between scopes when possible.

2. When you like to provide an explicit array, it will work as well. E.g.

   [1,2,3,4].map((rank, i, arr) => {
       if (arr.length - 1 === i) {
           // last one
       } else {
           // not last one
       }
   });
   

3. If you like to move the callback outside of the map scope (mainly for a better performance), it will be wrong to use row.length as it is out of scope. E.g. in the OP case:

   const mapElement = (rowIndex, state, toggled, onClick) => {
       return (rank, i, arr) => {
           let lastIndex = arr.length - 1;
           return [element(rowIndex, i, state, rank, toggled, onClick, lastIndex)];
       };
   };
   
   map = ranks.map((row, r) => row.map(mapElement(r, state, toggled, onClick)));
   

Fewer lines of code with the same results

row.map((rank, i, {length}) => (

  //last element
  if(i + 1 === length){ 

  }
));

A slight improvement on the accepted answer:

const lastIndex = row.length - 1;
row.map((rank, i) => {
  if (i === lastIndex) {
    // last one
  } else {
    // not last one
  }
})

This removes the arithmetic from inside the loop.