Understanding Array.slice() method in Javascript

Table of Contents

• Introduction
• Syntax
• The start index is undefined
• The start index is negative
• The start index is greater than the end_index
• The end index is omitted
• The end index is negative
• The end index is greater than the length of the array

Introduction

Array.slice() method returns a shallow copy of a part of an array into a new Array Object.

The part is selected on the basis of the start index and the end index passed as a parameter to the slice method.

Syntax

arr.slice([start_index[, end_index]])

start_index: specifies the start index from where the array is selected for copying. end_index: specifies the end index until where the array will be copied. The element at end_index is not selected.

Note: The original array is not modified by the slice method.

Example:

let arr = [ 0, 1, 2, 3, 4, 5 ];
arr.slice(0,3);
// [ 0, 1, 2 ]
// The element at the end_index is not selected

Let's learn about the slice method with a few more examples.

The start index is undefined

When the start_index is undefined, start_index will be taken as 0.

let arr = [0,1,2,3,4,5];
start_index = undefined;
end_index = 3
arr.slice(start_index, end_index);
// [0, 1, 2]

The start index is negative

The negative start_index will be used as an offset from the end of the original array and the element is taken as the start.

let arr = [ 0, 1, 2, 3, 4, 5 ];
start_index = -4;
end_index = 4;
arr.slice(start_index, end_index);

/* Counting 4 (the offset) from last of 
the original array is the element at index 2.*/

//last element that will be selected is at index before the end index ie. at index 3 of the original array. 

// output : [2, 3]

The start index is greater than the end_index

If the start_index is greater than the end_index, an empty array will be returned by the slice method.

let arr = [ 0, 1, 2, 3, 4, 5 ];
start_index = 4;
end_index = 2;
arr.slice(start_index, end_index);
// []

The end index is omitted

When the end_index is not specified, the array will be selected till the end.

let arr = [ 0, 1, 2, 3, 4, 5 ];
start_index = 4;
arr.slice(start_index);
// [4, 5]

The end index is negative

As seen for the start_index, if the end_index is negative, it will be treated as an offset from the end of the array for the last element that needs to be copied to the new array.

let arr = [ 0, 1, 2, 3, 4, 5 ];
start_index = 1;
end_index = -2;
arr.slice(start_index, end_index);

/* Counting 2 (the offset) from last of 
the original array is the element at index 4.*/

//The array will be selected from the index 1. 
// output : [1, 2, 3]

The end index is greater than the length of the array

When the end_index is greater than the end index of the array, the array will be selected until the last element for copying.

let arr = [ 0, 1, 2, 3, 4, 5 ];
start_index = 1;
end_index = 8;
arr.slice(start_index, end_index);
//[1, 2, 3, 4, 5]