FormData

`FormData.prototype.set`

Sets a new value for an existing key, or adds the key/value if it does not already exist, unlike append which adds another value

Syntax

JavaScript

formData.set(name, value, filename?)

Parameters

Parameter	Type	Description
name	string	The name of the field
value	string \| Blob	The field value
filename	string	Optional filename for Blob values

Return Value

undefined

Examples

Basic Usage

const fd = new FormData()
fd.set('name', 'Alice')
fd.set('name', 'Bob')
console.log(fd.get('name')) // 'Bob'

Practical Example

const form = document.querySelector('form') as HTMLFormElement
const fd = new FormData(form)
fd.set('timestamp', new Date().toISOString())

Advanced Usage

function updateField(fd: FormData, key: string, value: string) {
  fd.set(key, value)
  return fd
}

Understanding FormData.prototype.set

The FormData.prototype.set method in JavaScript sets a new value for an existing key, or adds the key/value if it does not already exist, unlike append which adds another value. It belongs to the FormData object and is one of the most widely used methods for working with formdata values in modern JavaScript and TypeScript applications.

The method signature is formData.set(name, value, filename?). It accepts 3 parameters: name, value, filename. When called, it returns undefined. Understanding when and how to use set() helps you write more expressive, readable code.

Common use cases for FormData.prototype.set include data transformation, input validation, API response processing, and building reusable utility functions. It works well alongside related methods like fetch-formdata-append, fetch-formdata-get, fetch-formdata-delete, enabling you to chain operations together for complex data manipulation pipelines.

Browser support for FormData.prototype.set is excellent across all modern browsers including Chrome, Firefox, Safari, and Edge. It is also fully supported in Node.js and Deno. For older environments, transpilation with Babel or a polyfill may be needed.