Javascript Map Array Last Item

问题:

I have this:

map = ranks.map((row, r) => (
  row.map((rank, i) => {
    return [element(r, i, state, rank, toggled, onClick)];
  })
));

It maps through a 2-dimentional array. After each row, I'd like to insert <div class="clearfix"></div>.

I think, if I could somehow get the last index for each row, so I will be able to use it in the row map callback. Can someone show me how to do it?

回答1:

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

回答2:

As LeoYuan 袁力皓 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
    }
});

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)));
   

回答3:

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.

回答4:

you can check last index with your array's length. here is a logic

var randomnumber = Math.floor(Math.random() * (100 - 10 + 1)) + 10

console.log("your last index is dynamic, ehich is ",randomnumber-1);
let arry = [];
for (i=1;i<randomnumber;i++){
    arry.push(i)
}

    arry.map((data,index)=>{
          if(index == arry.length-1 ){
            console.log("last index data  ",data)
          }
          else{
          console.log("remain data ",data)
          
          }

    })

    console.log("your last index is dynamic, which is ",randomnumber-1);

this is also works in dynamic arry changes.. it is a too simple technique which i use .. :-)

回答5:

simplify answer above

const array = ['apple','orange','banana'];

array.map((element, index) => (index === array.length - 1) ? \`${element}.\` : \`${element},\`);

回答6:

const array = ['apple','orange','banana'];

array.map((element, index) => {
  //Last element
  if (index === array.length - 1) {
    return `${element}.`;
  }
  //Another elements
  return `${element}, `;
})}

Will return apple, orange, banana.