All godot types (Object & Primitive) are defined in the built-in module godot in javascript. Most of them are lazily loaded until actually used in scripts.

Godot Object Types

Enum

Godot enums are exposed in javascript as enum, and the underlying constant values which already defined in enums are eliminated from bindings.

[!NOTE] The name of enum value can be retrieved by reading from enum type with the value as the key.

For example:

import { Image } from "godot";

// read enum value from the nested enum in class
let mode: Image.AlphaMode = Image.AlphaMode.ALPHA_NONE;

// can not read enum constant value from class
// Image.ALPHA_NONE is not defined in GodotJS.

// this statement prints '0'
console.log(mode); 

// this statement prints 'ALPHA_NONE'
console.log(Image.AlphaMode[mode]);

Godot Primitive Types (Variant)

Godot Variant values are automatically translated to the primitive types in javascript if available.

Variant Type in Godot	Counterpart in JS	Caveats
NIL	any	NIL used as a type stands for any variant type parameter in Godot
INT	number	INT is represented as int64 in Godot, therefore only values less than the max value of int32 can be translated without loss.
FLOAT	number
STRING	string
STRING_NAME	string

All variant types not mentioned above are exposed as class in javascript (such as Vector2 Vector3 PackedStringArray etc.).

[!NOTE] All primitive types which represented as Variant are pooled in Environment if JSB_WITH_VARIANT_POOL is enabled to reduce unnecessary cost on repeatedly memory allocation.

Pitfalls

The javascript class variant values behave like value type when passing between JS and C++, they are NOT by-reference. But in pure javascript, they are still by-reference.

let value = new Vector3();

// Input get a copy of the value since it's implemented in C++
Input.set_gyroscope(value); 

function modify(v: Vector3) {
    v.x = 1;
}

// after this statement, `value.x` will be 1, since value itself is passed by-reference
modify(value); 

// node.position is NOT modified, since `node.position` is actually a getter implemented in C++ which returns a copy of it.
modify(node.position); 

And, avoid coding property assignments like this: node.position.x = 0;, although, it works in GDScript.

It's not an error in javascript (which is more DANGEROUS), the actually modifed value is just a copy of node.position.

StringName

StringName optimization is completely transparent in javascript. When passing StringName parameters, a mapping between StringName and v8::String will be automatically cached to avoid repeatedly allocation of new v8::String objects and Godot Variant objects.

import { Input, Node2D } from 'godot';

class MyActor extends Node2D {
    _process() {
        // so it's relatively lightweight to directly use a string here
        if (Input.is_action_just_pressed("confirm")) {
            // ...
        }
    }
}

Packed Array (Vector<T>)

If JSB_IMPLICIT_PACKED_ARRAY_CONVERSION is defined as true, Packed Array will be implicitly converted from javascript Array.

let a1 = new PackedStringArray();
let a2 = new PackedStringArray();
a2.append("test2");

a1.append_array(["hey", "there"]); // implicit, it will fail if `JSB_IMPLICIT_PACKED_ARRAY_CONVERSION` is `0`
a1.append_array(a2); // explicit

// To get value from PackedArray use `get_indexed`
a1.get_indexed(2);

// To set value in PackedArray use `set_indexed`
a1.set_indexed(2, "new value");

let b1 = new PackedByteArray();
b1.append_array([20, 1, 6, 5]); 

// ArrayBuffer is implicitly treated as PackedByteBuffer even if `JSB_IMPLICIT_PACKED_ARRAY_CONVERSION` is `0`
// See `PackedByteArray and ArrayBuffer`
b1.append_array(new ArrayBuffer(16)); 

PackedByteArray and ArrayBuffer

A javascript ArrayBuffer can be used as a PackedByteArray implicitly.

let file = FileAccess.open(filePath, FileAccess.ModeFlags.WRITE);
let buffer = new ArrayBuffer(16);
let view = new Uint8Array(buffer);

view.fill(0);
file.store_buffer(buffer);

But reversely, PackedByteArray.to_array_buffer is required to get an ArrayBuffer from PackedByteArray.

let packed = FileAccess.get_file_as_bytes("res://something.txt");

// 'packed' is a `PackedByteArray`
console.log(packed.size()); 

// 'buffer' is a `ArrayBuffer`
let buffer = packed.to_array_buffer(); 
console.log(buffer.byteLength);