Function swc_ecma_transforms_compat::class_fields_use_set::class_fields_use_set

pub fn class_fields_use_set(pure_getters: bool) -> impl Fold + VisitMut

What does this module do?

This module will transpile the class semantics from [[Define]] to [[Set]].

Note: class’s native field is [[Define]] semantics.

Why is it needed?

The getter/setter from the super class won’t be triggered in [[Define]] semantics.

Some decorators depend on super class getter/setter. Therefore, scenarios like this will require [[Set]] semantics.

Example

class Foo {
    a = 1;
    #b = 2;
    static c = 3;
    static #d = 4;
}

result:

class Foo {
    #b;
    static {
        this.c = 3;
    }
    static #d = 4;
    constructor() {
        this.a = 1;
        this.#b = 2;
    }
}

The variable a will be relocated to the constructor. Although the variable #b is not influenced by [[Define]] or [[Set]] semantics, its execution order is associated with variable a, thus its initialization is moved into the constructor.

The static variable c is moved to the static block for [[Set]] semantic conversion. Whereas, variable #d remains completely unaffected and conserved in its original location.

Furthermore, for class props that have side effects, an extraction and conversion will be performed.

For example,

class Foo {
    [foo()] = 1;
}

result:

let prop;
class Foo{
    static {
        prop = foo();
    }
    constructor() {
        this[prop] = 1;
    }
}