splice and slice
- Node.js course
- Beginner
- Lesson 57/116
- 5 min
Learn array slicing with .splice and .slice.
Modifying with splice
To remove or insert elements from the middle of an array you use .splice(start, deleteCount, ...items). start is the start index, deleteCount is how many to remove, items are the elements to insert.
For example const a = [1, 2, 3, 4]; a.splice(1, 2); removes 2 elements starting at index 1; a becomes [1, 4]. It returns the removed elements as an array. To insert, use the third argument: a.splice(1, 0, 9) inserts 9 at index 1, removing nothing. splice modifies the array in place.
Copying with slice
To take a part of an array as a copy you use .slice(start, end). start is the start index, end is the end index; end is excluded. The original array is not modified; a new array is returned.
For example const a = [1, 2, 3, 4]; const part = a.slice(1, 3); makes part equal to [2, 3]; a stays [1, 2, 3, 4]. slice also supports negative indices: a.slice(-2) returns the last two elements. Unlike splice, slice does not touch the array; this is why it is preferred when you want to keep the data intact while taking a part.
Practice Take a part with slice
The nums variable is set to [1, 2, 3, 4]. Declare a variable part and assign the result of nums.slice(1, 3) to it. Then print part[0] with console.log. The output on a single line must be 2.
Starter code
const nums = [1, 2, 3, 4];
// Declare part with slice(1, 3); print its first elementSample solution
const nums = [1, 2, 3, 4];
const part = nums.slice(1, 3);
console.log(part[0]);Expected output
2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the result of const a = [1, 2, 3, 4]; a.splice(1, 2);?
const a = [1, 2, 3, 4];
a.splice(1, 2);- a = [1, 4] Correct answer
- a = [2, 3, 4]
- a = [1, 2, 3, 4]
- a = [1, 2]
Explanation: splice(1, 2) removes 2 elements from index 1; a becomes [1, 4].
2 Match each method to its job
| Term | Meaning |
|---|---|
splice |
Modifies in place |
slice |
Returns a copy part |
push |
Appends at end |
shift |
Removes from start |
3 Write the slice call
const a = [1, 2, 3, 4];
const part = a____(1, 3);const a = [1, 2, 3, 4];
const part = a.slice(1, 3);Explanation: .slice returns a copy of a part of the array.