append

Append a value to a key

Description

If key already exists and is a string, this command appends the value at the end of the string. If key does not exist, it is created and set as an empty string, so APPEND will be similar to SET in this special case.

Syntax

redis.append(key: string, value: string): Promise<number>

Parameters

key · string · required

The key to append to

value · string · required

The string value to append

Returns

length · number

The length of the string after the append operation

Examples

Basic Usage

import { Redis } from '@upstash/redis';

const redis = new Redis({
  url: 'https://your-redis-url.upstash.io',
  token: 'your-token'
});

// Append to a new key
const length1 = await redis.append('message', 'Hello');
console.log(length1); // 5

const length2 = await redis.append('message', ' World');
console.log(length2); // 11

const value = await redis.get('message');
console.log(value); // "Hello World"

Building Strings Incrementally

// Build a log message incrementally
await redis.append('log:request:123', '[2024-03-03] ');
await redis.append('log:request:123', 'User logged in ');
await redis.append('log:request:123', 'from IP 192.168.1.1');

const log = await redis.get('log:request:123');
console.log(log);
// "[2024-03-03] User logged in from IP 192.168.1.1"

Append to Non-Existent Key

// Appending to a non-existent key creates it
const length = await redis.append('newkey', 'First value');
console.log(length); // 11

const value = await redis.get('newkey');
console.log(value); // "First value"

Building CSV-like Data

// Create a simple CSV row
await redis.append('csv:row:1', 'Alice');
await redis.append('csv:row:1', ',30');
await redis.append('csv:row:1', ',Engineer');

const row = await redis.get('csv:row:1');
console.log(row); // "Alice,30,Engineer"

Notes

• APPEND is very efficient for building strings incrementally
• The time complexity is O(1), assuming the appended value is small and the already present value is of any size
• If the key exists but is not a string, an error is returned

Related Commands

• set - Set the string value of a key
• get - Get the value of a key
• setrange - Overwrite part of a string at a specific offset
• getrange - Get a substring of the string stored at a key

Redis Documentation

For more information, see the Redis APPEND documentation.