JavaScript For Scratch

This extension allows you to use various JavaScript features in Scratch!

Installation

To use ScratchJS, create a bookmarklet with the following code.
(For non-technical users: Make a bookmark and set the URL to this code.)

javascript: (async () => {
  try {
    const r = await fetch(
      "https://raw.githubusercontent.com/Ironbill25/JavaScript-For-Scratch/refs/heads/main/scratchjs.js",
      { cache: "no-cache" }
    );
    if (!r.ok) throw new Error("Fetch failed: " + r.status);
    const scriptText = await r.text();
    const s = document.createElement("script");
    s.textContent = scriptText;
    document.body.appendChild(s);
    console.log("Loaded");
  } catch (e) {
    console.error("Failed to load script:", e);
  }
})();

Go to the Scratch editor and click the bookmarklet to load the extension.

Safety Notice

When ScratchJS loads, you will see a warning modal indicating that this extension has access to advanced features. Only use projects with ScratchJS from creators you trust, as projects can potentially perform dangerous operations.

Block Categories

Math & Numbers

`[base] ^ [exponent]`

Type: Reporter
Returns: Base raised to the power of exponent

Parameters:

base (number, default: 2) - Base number
exponent (number, default: 3) - Exponent

Example: 2 ^ 3 returns 8

`Clamp [value] between [min] and [max]`

Type: Reporter
Returns: Value constrained within min and max range

Parameters:

value (number, default: 15) - Value to clamp
min (number, default: 0) - Minimum value
max (number, default: 10) - Maximum value

Example: Clamp 15 between 0 and 10 returns 10

`Round [number] to [decimals] decimal places`

Type: Reporter
Returns: Number rounded to specified decimal places

Parameters:

number (number, default: 3.14159) - Number to round
decimals (number, default: 2) - Number of decimal places

Example: Round 3.14159 to 2 decimal places returns 3.14

`[part]% of [whole]`

Type: Reporter
Returns: Percentage calculation

Parameters:

part (number, default: 25) - Percentage value
whole (number, default: 100) - Whole value

Example: 25% of 100 returns 25

`[value]++`

Type: Reporter
Returns: Value incremented by 1

Parameters:

value (number, default: 5) - Value to increment

Example: 5++ returns 6

`[value]--`

Type: Reporter
Returns: Value decremented by 1

Parameters:

value (number, default: 5) - Value to decrement

Example: 5-- returns 4

`[value1] >= [value2]`

Type: Boolean
Returns: True if value1 is greater than or equal to value2

Parameters:

value1 (number, default: 5) - First value
value2 (number, default: 5) - Second value

`[value1] <= [value2]`

Type: Boolean
Returns: True if value1 is less than or equal to value2

Parameters:

value1 (number, default: 5) - First value
value2 (number, default: 5) - Second value

`[value1] ≠ [value2]`

Type: Boolean
Returns: True if values are not equal

Parameters:

value1 (number, default: 5) - First value
value2 (number, default: 5) - Second value

Boolean & Constants

`True`

Type: Boolean
Returns: Always returns true

`False`

Type: Boolean
Returns: Always returns false

String & Text Manipulation

`Replace all [string] in [text] with [replace]`

Type: Reporter
Returns: Text with all occurrences of string replaced

Parameters:

text (string, default: "Hello World") - Original text
string (string, default: "World") - String to replace
replace (string, default: "Scratch") - Replacement string

Example: Replace all "World" in "Hello World" with "Scratch" returns "Hello Scratch"

`Get substring of [text] from [start] to [end]`

Type: Reporter
Returns: Portion of text between specified positions (1-based indexing)

Parameters:

text (string, default: "Hello World") - Original text
start (number, default: 1) - Start position (1-based)
end (number, default: 6) - End position (1-based)

Example: Get substring of "Hello World" from 1 to 6 returns "Hello"

`Reverse string [text]`

Type: Reporter
Returns: Text reversed character by character

Parameters:

text (string, default: "Hello World") - Text to reverse

Example: Reverse string "Hello" returns "olleH"

`Convert [text] to case [caseType]`

Type: Reporter
Returns: Text converted to specified case

Parameters:

text (string, default: "Hello World") - Text to convert
caseType (menu) - Case conversion type:
- UPPERCASE - Convert to all uppercase
- lowercase - Convert to all lowercase

Example: Convert "hello world" to case UPPERCASE returns "HELLO WORLD"

`Newline`

Type: Reporter
Returns: Newline character (\n)

`Tab`

Type: Reporter
Returns: Tab character (\t)

Date & Time

`current [format]`

Type: Reporter
Returns: Current date/time in specified format

Options:

date and time - Full date and time string
date only - Current date (locale format)
time only - Current time (locale format)
timestamp - Unix timestamp in milliseconds

Example: current date and time returns "3/17/2026, 8:06:00 PM"

`Current project ID`

Type: Reporter
Returns: The current Scratch project ID from the URL

Example: If URL is https://scratch.mit.edu/projects/123456/, returns "123456"

System Information

`Get info on the [what]`

Type: Reporter
Returns: Various system and browser information

Options:

operating system - OS platform (e.g., "Win32")
browser - Browser user agent string
language - Browser language (e.g., "en-US")
time zone - User's timezone (e.g., "America/New_York")
screen width - Screen width in pixels
screen height - Screen height in pixels
window width - Browser window width in pixels
window height - Browser window height in pixels
device pixel ratio - Device pixel ratio

Example: Get info on the operating system returns "Win32"

JavaScript Execution

`JS| Run JS code [code]`

Type: Command
Action: Executes the given JavaScript code

Parameters:

code (string, default: "alert('Hello World!')") - JavaScript code to execute

Warning: This block can execute any JavaScript code and should be used with caution.

`JS| Get return value of [code]`

Type: Reporter
Returns: The return value of the executed JavaScript code

Parameters:

code (string, default: "6473 / 84") - JavaScript code to execute and return value from

Example: JS| Get return value of "Math.random()" returns a random number between 0 and 1

`JS| Set variable [name] to [val]`

Type: Command
Action: Creates or updates a JavaScript variable

Parameters:

name (string, default: "window.example") - Variable name
val (string, default: "Hello World!") - Value to assign

Example: JS| Set variable "window.myVar" to "test" creates a global variable

File & Web Operations

`JS| Open site [url]`

Type: Command
Action: Opens the specified URL in a new browser tab

Parameters:

url (string, default: "https://example.com") - URL to open

Example: JS| Open site "https://scratch.mit.edu" opens Scratch in a new tab

`JS| Open this project in Turbowarp`

Type: Command
Action: Opens the current project in Turbowarp

`JS| Reload page`

Type: Command
Action: Reloads the current page

`JS| Save file [name] with contents [contents]`

Type: Command
Action: Downloads a text file with the specified name and contents

Parameters:

name (string, default: "example.txt") - Filename
contents (string, default: "Hello World!") - File contents

Example: JS| Save file "data.txt" with contents "Hello from Scratch!" downloads a text file

Control Flow & Loops

`when [condit] is true`

Type: Hat
Triggers: When the condition becomes true

Parameters:

condit (boolean) - Condition to monitor

Example: when "mouse down?" is true triggers when mouse is pressed

`For i in [value]`

Type: Conditional
Action: Loop control block that increments i and checks if i <= value

Parameters:

value (string, default: "10") - Maximum value for loop

Use: Works with the i reporter and set i blocks for custom loops

`i`

Type: Reporter
Returns: Current value of loop counter i

`Set i to [value]`

Type: Command
Action: Sets the loop counter i to specified value

Parameters:

value (number, default: 0) - Value to set i to

Mouse & Input

`Mouse X (works out of bounds)`

Type: Reporter
Returns: Mouse X coordinate (works even when mouse is outside stage)

`Mouse Y (works out of bounds)`

Type: Reporter
Returns: Mouse Y coordinate (works even when mouse is outside stage)

`Mouse down? (works out of bounds)`

Type: Boolean
Returns: True if mouse button is pressed (works even when mouse is outside stage)

Boolean & Type Conversion

`[bool]`

Type: Boolean
Returns: Boolean value converted from text

Parameters:

bool (string, default: "true") - Text to convert to boolean

Accepts: "true", "1", "True", or any non-empty string (except "0", "false", "False") returns true

`[bool]`

Type: Reporter
Returns: Text representation of boolean value

Parameters:

bool (boolean) - Boolean to convert to text

Example: Converts true to "true" and false to "false"

Utility Blocks

`[arg1]`

Type: Reporter
Returns: The input argument unchanged

Parameters:

arg1 (string, default: "Hello") - Input value

Use: Useful for type conversion or passing values through blocks

`if [arg1] then [arg2] else [arg3]`

Type: Reporter
Returns: arg2 if arg1 is true, otherwise arg3

Parameters:

arg1 (boolean) - Condition
arg2 (string, default: "Hello") - Value if true
arg3 (string, default: "World") - Value if false

Example: if "mouse down?" then "Pressed" else "Released" returns "Pressed" when mouse is down

Array Operations

`Blank array`

Type: Reporter
Returns: Empty array as JSON string "[]"

`Append [value] to array [array]`

Type: Reporter
Returns: Array with value appended to end

Parameters:

value (string, default: "Hello") - Value to append
array (string, default: "[]") - Array as JSON string

Example: Append "World" to array "[\"Hello\"]" returns "["Hello","World"]"

`Get [index] from array [array]`

Type: Reporter
Returns: Item at specified index (1-based)

Parameters:

index (number, default: 1) - Index (1-based)
array (string, default: "[]") - Array as JSON string

Example: Get 2 from array "[\"Apple\",\"Banana\"]" returns "Banana"

`Insert [value] at [index] in array [array]`

Type: Reporter
Returns: Array with value inserted at specified position

Parameters:

value (string, default: "Hello") - Value to insert
index (number, default: 1) - Index position (1-based)
array (string, default: "["Apple"]") - Array as JSON string

`Replace [index] in array [array] with [value]`

Type: Reporter
Returns: Array with item at index replaced

Parameters:

index (number, default: 1) - Index to replace (1-based)
array (string, default: "["Apple"]") - Array as JSON string
value (string, default: "Banana") - New value

`Remove [index] from array [array]`

Type: Reporter
Returns: Array with item at specified index removed

Parameters:

index (number, default: 1) - Index to remove (1-based)
array (string, default: "["Apple"]") - Array as JSON string

`Merge [array1] and [array2]`

Type: Reporter
Returns: Two arrays combined into one

Parameters:

array1 (string, default: "["Hello"]") - First array
array2 (string, default: "["World"]") - Second array

`Length of array [array]`

Type: Reporter
Returns: Number of items in array

Parameters:

array (string, default: "["Apple", "Banana"]") - Array as JSON string

`Array [array] contains [value]`

Type: Boolean
Returns: True if array contains the specified value

Parameters:

array (string, default: "["Apple", "Banana"]") - Array as JSON string
value (string, default: "Carrot") - Value to search for

`Index of [value] in array [array]`

Type: Reporter
Returns: Index of first occurrence of value (1-based, returns 0 if not found)

Parameters:

value (string, default: "Hello") - Value to find
array (string, default: "["Apple"]") - Array as JSON string

`Split [string] by [delimiter] into array`

Type: Reporter
Returns: String split into array by delimiter

Parameters:

string (string, default: "Hello, World") - String to split
delimiter (string, default: ",") - Delimiter character

Example: Split "Hello, World" by "," into array returns "["Hello"," World"]"

`Join array [array] with [delimiter]`

Type: Reporter
Returns: Array elements joined into string with delimiter

Parameters:

array (string, default: "["Hello", "World"]") - Array as JSON string
delimiter (string, default: ",") - Delimiter character

Example: Join array "[\"Hello\",\"World\"]" with "," returns "Hello,World"

Object Operations

`Blank object`

Type: Reporter
Returns: Empty object as JSON string "{}"

`Set [key] in object [object] to [value]`

Type: Reporter
Returns: Object with key set to value

Parameters:

key (string, default: "name") - Property key
object (string, default: "{}") - Object as JSON string
value (string, default: "John") - Value to set

Example: Set "name" in object "{}" to "John" returns "{"name":"John"}"

`Get [key] from object [object]`

Type: Reporter
Returns: Value of specified key from object

Parameters:

key (string, default: "name") - Property key
object (string, default: "{"name": "John"}") - Object as JSON string

Example: Get "name" from object "{\"name\": \"John\"}" returns "John"

`Delete [key] from object [object]`

Type: Reporter
Returns: Object with specified key removed

Parameters:

key (string, default: "name") - Property key to delete
object (string, default: "{"name": "John"}") - Object as JSON string

`Object [object] has key [key]`

Type: Boolean
Returns: True if object contains the specified key

Parameters:

object (string, default: "{"name": "John"}") - Object as JSON string
key (string, default: "name") - Key to check

`Keys of object [object] (array)`

Type: Reporter
Returns: Array of object's keys as JSON string

Parameters:

object (string, default: "{"name": "John"}") - Object as JSON string

Example: Keys of object "{\"name\": \"John\"}" (array) returns "["name"]"

`Values of object [object] (array)`

Type: Reporter
Returns: Array of object's values as JSON string

Parameters:

object (string, default: "{"name": "John"}") - Object as JSON string

Example: Values of object "{\"name\": \"John\"}" (array) returns "["John"]"

`Entries of object [object] (array)`

Type: Reporter
Returns: Array of [key, value] pairs as JSON string

Parameters:

object (string, default: "{"name": "John"}") - Object as JSON string

Example: Entries of object "{\"name\": \"John\"}" (array) returns "[["name","John"]]"

`Size of object [object]`

Type: Reporter
Returns: Number of keys in object

Parameters:

object (string, default: "{"name": "John"}") - Object as JSON string

`Get path (array) [path] from object [object]`

Type: Reporter
Returns: Value at nested path in object

Parameters:

path (string, default: "["name"]") - Path as JSON array
object (string, default: "{"name": "John"}") - Object as JSON string

Example: Get path "[\"name\"]" from object "{\"name\": \"John\"}" returns "John"

`Set path (array) [path] in object [object] to [value]`

Type: Reporter
Returns: Object with nested path set to value

Parameters:

path (string, default: "["name"]") - Path as JSON array
object (string, default: "{}") - Object as JSON string
value (string, default: "John") - Value to set

Example: Set path "[\"name\"]" in object "{}" to "John" returns "{"name":"John"}"

Technical Details

Block Types Used

REPORTER: Returns a value, can be used in round inputs
COMMAND: Performs an action, can be stacked
HAT: Event-triggered block, runs code below when condition is met
BOOLEAN: Returns true/false, can be used in boolean inputs

Argument Types Used

string: Accepts any text or reporter block
number: Accepts numbers or reporter blocks
Boolean: Accepts only boolean blocks
menu: Dropdown menu with predefined options

Security Considerations

ScratchJS provides powerful JavaScript execution capabilities. Users should:

Only use projects from trusted creators
Be cautious with blocks that execute arbitrary JavaScript
Understand that file operations and web access are available

Development

ScratchJS is built using the Scratch extension system. The extension:

Waits for the Scratch VM to be available
Creates a new ScratchJS extension instance
Registers all blocks with the Scratch runtime
Provides JavaScript execution environment

For more information on creating Scratch extensions, see extensiontutorial.md.

Name		Name	Last commit message	Last commit date
Latest commit History 83 Commits
scratchjsblocks		scratchjsblocks
LICENSE		LICENSE
README.md		README.md
archived.js.txt		archived.js.txt
extensiontempl.js		extensiontempl.js
extensiontutorial.md		extensiontutorial.md
scratchjs.js		scratchjs.js

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Latest commit

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JavaScript For Scratch

Installation

Safety Notice

Block Categories

Math & Numbers

[base] ^ [exponent]

Clamp [value] between [min] and [max]

Round [number] to [decimals] decimal places

[part]% of [whole]

[value]++

[value]--

[value1] >= [value2]

[value1] <= [value2]

[value1] ≠ [value2]

Boolean & Constants

True

False

String & Text Manipulation

Replace all [string] in [text] with [replace]

Get substring of [text] from [start] to [end]

Reverse string [text]

Convert [text] to case [caseType]

Newline

Tab

Date & Time

current [format]

Current project ID

System Information

Get info on the [what]

JavaScript Execution

JS| Run JS code [code]

JS| Get return value of [code]

JS| Set variable [name] to [val]

File & Web Operations

JS| Open site [url]

JS| Open this project in Turbowarp

JS| Reload page

JS| Save file [name] with contents [contents]

Control Flow & Loops

when [condit] is true

For i in [value]

i

Set i to [value]

Mouse & Input

Mouse X (works out of bounds)

Mouse Y (works out of bounds)

Mouse down? (works out of bounds)

Boolean & Type Conversion

[bool]

[bool]

Utility Blocks

[arg1]

if [arg1] then [arg2] else [arg3]

Array Operations

Blank array

Append [value] to array [array]

Get [index] from array [array]

Insert [value] at [index] in array [array]

Replace [index] in array [array] with [value]

Remove [index] from array [array]

Merge [array1] and [array2]

Length of array [array]

Array [array] contains [value]

Index of [value] in array [array]

Split [string] by [delimiter] into array

Join array [array] with [delimiter]

Object Operations

Blank object

Set [key] in object [object] to [value]

Get [key] from object [object]

Delete [key] from object [object]

Object [object] has key [key]

Keys of object [object] (array)

Values of object [object] (array)

Entries of object [object] (array)

Size of object [object]

`[base] ^ [exponent]`

`Clamp [value] between [min] and [max]`

`Round [number] to [decimals] decimal places`

`[part]% of [whole]`

`[value]++`

`[value]--`

`[value1] >= [value2]`

`[value1] <= [value2]`

`[value1] ≠ [value2]`

`True`

`False`

`Replace all [string] in [text] with [replace]`

`Get substring of [text] from [start] to [end]`

`Reverse string [text]`

`Convert [text] to case [caseType]`

`Newline`

`Tab`

`current [format]`

`Current project ID`

`Get info on the [what]`

`JS| Run JS code [code]`

`JS| Get return value of [code]`

`JS| Set variable [name] to [val]`

`JS| Open site [url]`

`JS| Open this project in Turbowarp`

`JS| Reload page`

`JS| Save file [name] with contents [contents]`

`when [condit] is true`

`For i in [value]`

`i`

`Set i to [value]`

`Mouse X (works out of bounds)`

`Mouse Y (works out of bounds)`

`Mouse down? (works out of bounds)`

`[bool]`

`[bool]`

`[arg1]`

`if [arg1] then [arg2] else [arg3]`

`Blank array`

`Append [value] to array [array]`

`Get [index] from array [array]`

`Insert [value] at [index] in array [array]`

`Replace [index] in array [array] with [value]`

`Remove [index] from array [array]`

`Merge [array1] and [array2]`

`Length of array [array]`

`Array [array] contains [value]`

`Index of [value] in array [array]`

`Split [string] by [delimiter] into array`

`Join array [array] with [delimiter]`

`Blank object`

`Set [key] in object [object] to [value]`

`Get [key] from object [object]`

`Delete [key] from object [object]`

`Object [object] has key [key]`

`Keys of object [object] (array)`

`Values of object [object] (array)`

`Entries of object [object] (array)`

`Size of object [object]`

`Get path (array) [path] from object [object]`

`Set path (array) [path] in object [object] to [value]`