Logic

Internally, Scratch follows certain rules and steps to perform operations. They are provided here for reference.

Casting

Most blocks automatically convert values between types for use using certain logic, also known as casting. The logic Scratch uses to perform most conversions is specified below.

To String

Scratch uses the following logic to convert any given value to a string:

• If the value is a string:
  • Return the value, as it is already a string.
• If the value is a number:
  • Follow the logic JavaScript does to cast a number to a string and return it.
    • TODO: specify this better
  • If the value is NaN, return the string NaN.
  • If the value is Infinity, return the string Infinity.
  • If the value is -Infinity, return the string -Infinity.
• If the value is a boolean:
  • If the value is true, return the string true.
  • If the value is false, return the string false.
• If the value is undefined, return the string undefined.
• If the value is something else, follow the logic JavaScript has for the toString method of that value to convert it to a string and return it. (Likely unnecessary though, as no known "other values" can be encountered.)
  • If the value is somehow null, return the string null.
  • TODO: Explain this step better or maybe just omit it because this step is not needed

To Number

Scratch uses the following logic to convert any given value to a number:

• If the value is NaN, return the number 0.
• If the value is a number, return the value as it is.
• If the value is undefined, return the number 0.
• If the value is true, return the number 1.
• If the value is false, return the number 0.
• If the value is a string or something else:
  • If the value can be converted to a number according to these rules, return it.
  • Otherwise, return the number 0.
  • TODO: specify string -> number coercion better b/c JavaScript logic

To Boolean

Scratch uses the following logic to convert any given value to a boolean:

Falsy

A value is falsy, or casted to false, if the value is:

• an empty string (no letters, length is 0),
• the strings 0 or false (case-insensitive),
• one of the numbers 0, -0, or NaN,
• undefined,
• null (rare),
• or false.

Truthy

Any value that is not falsy is considered truthy and casted to true.

To Direction

Scratch uses the following logic to convert any given value to a direction:

1. Cast the value to number.

2. Plug the casted value into the following equation as $a$ to find $d$:

   • $d=\mathrm{mod}(a+179,360)-179$

   • Here is the same equation written in blocks:

3. Return $d$ (direction).

Fencing

TODO: This section has an excessive amount of fluff and does not provide any actual information and will have to be rewritten... sometime

In Scratch, whenever the position of a sprite changes, it then fences (aka restricts) the position of the sprite to fit inside of a fence (aka boundary) that is intended to keep it within the viewing area of the stage. Additionally, whenever the size of a sprite changes, it then fences (aka restricts) the size of the sprite to remain reasonably visible and not too large so as to cause rendering problems. This is primarily so that sprites do not randomly disappear due to erroneous code, but it is also an intentional feature of Scratch that does get utilized by some projects.

Scratch uses the following procedures to determine what the position and size of a sprite should be limited to depending on the width and height of its current costume, as well as its direction.

Fencing Position

• TODO 😄

Fencing Size

• TODO ;D

Each fencing procedure is only applied after the position or size of the sprite changes (if the position changes, only the position is fenced; if the size changes, only the size is fenced). Because of this behavior, if one switches the costume to a particularly large or small costume, moves or resizes it, and then switches the costume back without changing the position and size afterward, then the sprite will remain at the same position and size that it had with the large or small costume (since fencing is not applied when switching costumes), allowing projects to circumvent fencing if needed.