js get free variables hack

get_free_vars.ts

/**
 * some insane hack, just want to avoid using expensive parser.
 */
export function getFreeVariables(expr:string, knownSymbols:Record<string,unknown>){

    const freeVariables = new Set<string>();

    //eslint-disable-next-line
    const anyThingPrimitive = ()=>{};

    Object.defineProperties(anyThingPrimitive, {
        [Symbol.toPrimitive]:{
            value: ()=>{
                return 1
            },
        },
        [Symbol.toStringTag]:{
            value: ()=>{
                return ""
            }
        }
    })

    const anyThing2:unknown = new Proxy(anyThingPrimitive,{
        get(target, prop){
            if(prop === Symbol.unscopables){
                return undefined
            }
            if(prop in target){
                return (target as unknown as Record<string,unknown>)[prop as string]
            }
            return anyThing2
        },
        apply(target, thisArg, args){
            return anyThing2
        }
    })

    const anyThing = new Proxy(anyThingPrimitive,{
        has(target, prop){
            if(prop === 'unsafe___THIS____func'){
                return false
            }
            if(prop === 'arguments'){
                return false
            }
            if(prop in knownSymbols){
                return false
            }
            return true;
        },
        get(target, prop){
            if(prop === Symbol.unscopables){
                return undefined
            }
            if(prop in target){
                return (target as unknown as Record<string,unknown>)[prop as string]
            }
            freeVariables.add(prop as string)
            return anyThing2
        },
        apply(target, thisArg, args){
            return anyThing2
        }
    })

    {
        const unsafe___THIS____func = new Function(`with (arguments[0]) { with (arguments[1]) { return ${expr} } }`);

        unsafe___THIS____func(anyThing, knownSymbols);
    }

    return [...freeVariables];

}