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My task is to split the given array into smaller arrays using JavaScript. For example [1, 2, 3, 4] should be split to [1] [1, 2] [1, 2, 3] [1, 2, 3, 4] [2] [2, 3] [2, 3, 4] [3] [3, 4] [4].

I am using this code:

let arr = [1, 2, 3, 4];

for (let i = 1; i <= arr.length; i++) 
 let a = ;
 for (let j = 0; j < arr.length; j++) 
 a.push(arr[j]);
 if (a.length === i) 
 break;
 
 
 console.log(a);

And I get the following result: [1] [1, 2] [1, 2, 3] [1, 2, 3, 4] undefined

What am I missing/doing wrong?

You have two issues in your code:

1. You need to have loop to initialize with the value of i for the inner loop so that it consider the next index for new iteration of i
   


2. You need to remove that break on the length which you have in inner loop.

let arr = [1, 2, 3, 4];
for (let i = 0; i <= arr.length; i++) 
 let a = ;
 for (let j = i; j < arr.length; j++) 
 a.push(arr[j]);
 console.log(a);
 

For the inner array, you could just start with the index of the outer array.

var array = [1, 2, 3, 4],
 i, j, l = array.length,
 result = ;
 
for (i = 0; i < l; i++) 
 for (j = i; j < l; j++) 
 result.push(array.slice(i, j + 1));
 

console.log(result.map(a => a.join(' ')));

.as-console-wrapper max-height: 100% !important; top: 0; 

Try this

 let arr = [1, 2, 3, 4];
 for (let i = 0; i <= arr.length; i++) 
 let a = ;
 for (let j = i; j < arr.length; j++) 
 a.push(arr[j]); 
 console.log(a); 
 
 

 let arr = [1, 2, 3, 4];
 let res = ;
 for (let i = 0; i <= arr.length; i++) 
 let a = ;
 for (let j = i; j < arr.length; j++) 
 a = [...a, arr[j]];
 res = [...res, a];
 
 
 console.log(res);

i have prepare stackblitz for this case.

let source = [1,2,3,4];
const output = ;
const arrayMultiplier = (source) => 
 const eachValueArray = ;
 source.forEach((item, index) => 
 // Will push new array who will be sliced source array.
 eachValueArray.push(source.slice(0, source.length - index));
 );
 //We reverse array to have right order.
 return eachValueArray.reverse();
;

for(let i = 0; i <= source.length; i++) 
 output.push(...arrayMultiplier(source));
 source.shift(); // Will recraft source array by removing first index.

//Don't forget last item.
output.push(source);
console.log(output);

Is not the most shorten solution but do the job

== update after code review ==

// [...]
const arrayMultiplier = (source) => 
 // Will push new array who will be sliced source array.
 // We reverse array to have right order.
 return source.map((item, index) => source.slice(0, source.length - index)).reverse();
;
// [...]