for / in循环递减javascript

Question

I am trying to decrement a for/in loop in js.

I have found lots of instruction on decrementing a for-loop, but had no luck finding a statement that decrements a for/in loop.

 var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"]; for( var index in classRegister ) { console.log(classRegister[index]); } 

This is my incrementing for/in loop statement that I want to modify to decrement.

Answer 1

Use basic for loop & avoid for..in to iterate an array

 var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"]; for (let i = classRegister.length - 1; i >= 0; i--) { console.log(classRegister[i]); } 

Answer 2

If you want to use for/in you can, you simply need to subtract your index from the last index value.

var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"];
for( var index in classRegister ) { 
 console.log(classRegister[classRegister.length - 1 - index]);
}

Answer 3

You should not use for..in for arrays instead use simple loop

 var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"]; for(let index = classRegister.length-1; index >= 0; index-- ) { console.log(classRegister[index]); } 

Or a while loop

 var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"]; let index = classRegister.length-1; while(index >= 0) { console.log(classRegister[index]); index--; } 

There are hacky ways to do but you should never use them IMO

 var classRegister = ["Lawrence", "John", "Jeff", "Bobbi"]; let copy = [...classRegister].reverse() // this reverse array in place for( var index in copy ) { console.log(copy[index]); }