diff --git a/wasm/function.go b/wasm/function.go
index 5b29f35..81ea4b9 100644
--- a/wasm/function.go
+++ b/wasm/function.go
@@ -83,13 +83,13 @@ func conformJSValueToType(funcType reflect.Type, this js.Value, values []js.Valu
 	in := make([]reflect.Value, 0, len(values))
 	for i, v := range values {
 		paramType := funcType.In(i)
-		x := reflect.Zero(paramType).Interface()
-		err := FromJSValue(v, &x)
+		ptrX := reflect.New(paramType).Interface()
+		err := FromJSValue(v, ptrX)
 		if err != nil {
 			return nil, err
 		}
 
-		in = append(in, reflect.ValueOf(x))
+		in = append(in, reflect.ValueOf(ptrX).Elem())
 	}
 
 	return in, nil
diff --git a/wasm/reflect_from.go b/wasm/reflect_from.go
index 8b119b7..2d23b37 100644
--- a/wasm/reflect_from.go
+++ b/wasm/reflect_from.go
@@ -1,13 +1,411 @@
 package wasm
 
 import (
+	"errors"
 	"fmt"
+	"reflect"
 	"syscall/js"
 )
 
+// ErrMultipleReturnValue is an error where a JS function is attempted to be unmarshalled into a Go function with
+// multiple return values.
+var ErrMultipleReturnValue = errors.New("a JS function can only return one value")
+
+// InvalidFromJSValueError is an error where an invalid argument is passed to FromJSValue.
+// The argument to Unmarshal must be a non-nil pointer.
+type InvalidFromJSValueError struct {
+	Type reflect.Type
+}
+
+// Error implements error.
+func (e InvalidFromJSValueError) Error() string {
+	return "invalid argument passed to FromJSValue. Got type " + e.Type.String()
+}
+
+// InvalidTypeError is an error where the JS value cannot be unmarshalled into the provided Go type.
+type InvalidTypeError struct {
+	JSType js.Type
+	GoType reflect.Type
+}
+
+// Error implements error.
+func (e InvalidTypeError) Error() string {
+	return "invalid unmarshalling: cannot unmarshal " + e.JSType.String() + " into " + e.GoType.String()
+}
+
+// InvalidArrayError is an error where the JS's array length do not match Go's array length.
+type InvalidArrayError struct {
+	Expected int
+	Actual   int
+}
+
+// Error implements error.
+func (e InvalidArrayError) Error() string {
+	return fmt.Sprintf(
+		"invalid unmarshalling: expected array of length %d to match Go array but got JS array of length %d",
+		e.Expected, e.Actual,
+	)
+}
+
+// Decoder is an interface which manually decodes js.Value on its own.
+// It overrides in FromJSValue.
+type Decoder interface {
+	FromJSValue(js.Value) error
+}
+
 // FromJSValue converts a given js.Value to the Go equivalent.
 // The new value of 'out' is undefined if FromJSValue returns an error.
+//
+// When a JS function is unmarshalled into a Go function with only one return value, the returned JS value is casted
+// into the type of the return value. If the conversion fails, the function call panics.
+//
+// When a JS function is unmarshalled into a Go function with two return values, the second one being error, the
+// conversion error is returned instead.
 func FromJSValue(x js.Value, out interface{}) error {
-	// TODO
-	return fmt.Errorf("unimplemented")
+	v := reflect.ValueOf(out)
+	if v.Kind() != reflect.Ptr || v.IsNil() {
+		return &InvalidFromJSValueError{reflect.TypeOf(v)}
+	}
+
+	return decodeValue(x, v.Elem())
+}
+
+// decodeValue decodes the provided js.Value into the provided reflect.Value.
+func decodeValue(x js.Value, v reflect.Value) error {
+	// If we have undefined or null, we need to be able to set to the pointer itself.
+	// All code beyond this point are pointer-unaware so we handle undefined or null first.
+	if x.Type() == js.TypeUndefined || x.Type() == js.TypeNull {
+		return decodeNothing(v)
+	}
+
+	// Implementations of Decoder are probably on pointer so do it before pointer code.
+	if d, ok := v.Addr().Interface().(Decoder); ok {
+		return d.FromJSValue(x)
+	}
+
+	// Make sure everything is initialized and indirect it.
+	// This prevents other decode functions from having to handle pointers.
+	if v.Kind() == reflect.Ptr {
+		initializePointerIfNil(v)
+		v = reflect.Indirect(v)
+	}
+
+	if v.Kind() == reflect.Interface && v.NumMethod() == 0 {
+		// It's a interface{} so we just create the easiest Go representation we can in createInterface.
+		res := createInterface(x)
+		if res != nil {
+			v.Set(reflect.ValueOf(res))
+		}
+		return nil
+	}
+
+	// Directly set v if it's a js.Value.
+	if _, ok := v.Interface().(js.Value); ok {
+		v.Set(reflect.ValueOf(x))
+		return nil
+	}
+
+	// Go the reflection route.
+	switch x.Type() {
+	case js.TypeBoolean:
+		return decodeBoolean(x, v)
+	case js.TypeNumber:
+		return decodeNumber(x, v)
+	case js.TypeString:
+		return decodeString(x, v)
+	case js.TypeSymbol:
+		return decodeSymbol(x, v)
+	case js.TypeObject:
+		if isArray(x) {
+			return decodeArray(x, v)
+		}
+		return decodeObject(x, v)
+	case js.TypeFunction:
+		return decodeFunction(x, v)
+	default:
+		panic("unknown JS type: " + x.Type().String())
+	}
+}
+
+// decodeNothing decodes an undefined or a null into the provided reflect.Value.
+func decodeNothing(v reflect.Value) error {
+	if v.Kind() != reflect.Ptr {
+		return InvalidTypeError{js.TypeNull, v.Type()}
+	}
+	v.Set(reflect.ValueOf(nil))
+	return nil
+}
+
+// decodeBoolean decodes a bool into the provided reflect.Value.
+func decodeBoolean(x js.Value, v reflect.Value) error {
+	if v.Kind() != reflect.Bool {
+		return InvalidTypeError{js.TypeBoolean, v.Type()}
+	}
+	v.SetBool(x.Bool())
+	return nil
+}
+
+// decodeNumber decodes a JS number into the provided reflect.Value, truncating as necessary.
+func decodeNumber(x js.Value, v reflect.Value) error {
+	switch v.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		v.SetInt(int64(x.Float()))
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		v.SetUint(uint64(x.Float()))
+	case reflect.Float32, reflect.Float64:
+		v.SetFloat(x.Float())
+	default:
+		return InvalidTypeError{js.TypeNumber, v.Type()}
+	}
+	return nil
+}
+
+// decodeString decodes a JS string into the provided reflect.Value.
+func decodeString(x js.Value, v reflect.Value) error {
+	if v.Kind() != reflect.String {
+		return InvalidTypeError{js.TypeString, v.Type()}
+	}
+	v.SetString(x.String())
+	return nil
+}
+
+// decodeSymbol decodes a JS symbol into the provided reflect.Value.
+func decodeSymbol(x js.Value, v reflect.Value) error {
+	// TODO Decode it into a symbol type.
+	return InvalidTypeError{js.TypeSymbol, v.Type()}
+}
+
+// decodeArray decodes a JS array into the provided reflect.Value.
+func decodeArray(x js.Value, v reflect.Value) error {
+	jsLen := x.Length()
+
+	switch v.Kind() {
+	case reflect.Array:
+		if jsLen != v.Len() {
+			return InvalidArrayError{v.Len(), jsLen}
+		}
+	case reflect.Slice:
+		newSlice := reflect.MakeSlice(v.Type(), jsLen, jsLen)
+		v.Set(newSlice)
+	default:
+		return InvalidTypeError{js.TypeObject, v.Type()}
+	}
+
+	for i := 0; i < jsLen; i++ {
+		err := FromJSValue(x.Index(i), v.Index(i).Addr().Interface())
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// decodeObject decodes a JS object into the provided reflect.Value.
+func decodeObject(x js.Value, v reflect.Value) error {
+	switch v.Kind() {
+	case reflect.Struct:
+		return decodeObjectIntoStruct(x, v)
+	case reflect.Map:
+		return decodeObjectIntoMap(x, v)
+	default:
+		return InvalidTypeError{js.TypeObject, v.Type()}
+	}
+}
+
+// decodeObject decodes a JS object into the provided reflect.Value struct.
+func decodeObjectIntoStruct(x js.Value, v reflect.Value) error {
+	for i := 0; i < v.Type().NumField(); i++ {
+		fieldType := v.Type().Field(i)
+		if fieldType.PkgPath != "" {
+			continue
+		}
+
+		name := fieldType.Name
+		tagName, tagOK := fieldType.Tag.Lookup("wasm")
+
+		if tagOK {
+			name = tagName
+		}
+
+		err := decodeValue(x.Get(name), v.Field(i))
+		if err != nil {
+			if tagOK {
+				return fmt.Errorf("in field %s (JS %s): %w", fieldType.Name, tagName, err)
+			}
+			return fmt.Errorf("in field %s: %w", fieldType.Name, err)
+		}
+	}
+
+	return nil
+}
+
+func decodeObjectIntoMap(x js.Value, v reflect.Value) error {
+	mapType := v.Type()
+	keyType := mapType.Key()
+	valType := mapType.Elem()
+
+	switch keyType.Kind() {
+	case reflect.String:
+	case reflect.Interface:
+		if keyType.NumMethod() != 0 {
+			return InvalidTypeError{js.TypeObject, mapType}
+		}
+	default:
+		return InvalidTypeError{js.TypeObject, mapType}
+	}
+
+	// TODO: Use Object API
+	obj, err := Global().Get("Object")
+	if err != nil {
+		panic("Object not found")
+	}
+
+	var keys []string
+	err = FromJSValue(obj.Call("keys", x), &keys)
+	if err != nil {
+		panic("Object.keys returned non-string-array.")
+	}
+
+	for _, k := range keys {
+		valuePtr := reflect.New(valType).Interface()
+		err := FromJSValue(x.Get(k), valuePtr)
+		if err != nil {
+			return err
+		}
+
+		v.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(valuePtr).Elem())
+	}
+	return nil
+}
+
+// decodeFunction decodes a JS function into the provided reflect.Value.
+func decodeFunction(x js.Value, v reflect.Value) error {
+	funcType := v.Type()
+	outCount := funcType.NumOut()
+
+	switch outCount {
+	case 0, 1:
+	case 2:
+		if funcType.Out(1) != errorType {
+			return ErrMultipleReturnValue
+		}
+	default:
+		return ErrMultipleReturnValue
+	}
+
+	v.Set(reflect.MakeFunc(funcType, func(args []reflect.Value) []reflect.Value {
+		argsJS := make([]interface{}, 0, len(args))
+		for _, v := range args {
+			argsJS = append(argsJS, ToJSValue(v.Interface()))
+		}
+
+		jsReturn := x.Invoke(argsJS...)
+		if outCount == 0 {
+			return []reflect.Value{}
+		}
+
+		returnPtr := reflect.New(funcType.Out(0)).Interface()
+		err := FromJSValue(jsReturn, returnPtr)
+
+		returnVal := reflect.ValueOf(returnPtr).Elem()
+		if err != nil {
+			if outCount == 1 {
+				panic("error decoding JS return value: " + err.Error())
+			}
+
+			return []reflect.Value{returnVal, reflect.ValueOf(err)}
+		}
+
+		switch outCount {
+		case 1:
+			return []reflect.Value{returnVal}
+		case 2:
+			return []reflect.Value{returnVal, reflect.ValueOf(nil)}
+		default:
+			panic("unexpected amount of return values")
+		}
+	}))
+	return nil
+}
+
+// createInterface creates a representation of the provided js.Value.
+func createInterface(x js.Value) interface{} {
+	switch x.Type() {
+	case js.TypeUndefined, js.TypeNull:
+		return nil
+	case js.TypeBoolean:
+		return x.Bool()
+	case js.TypeNumber:
+		return x.Float()
+	case js.TypeString:
+		return x.String()
+	case js.TypeSymbol:
+		// We can't convert it to a Go value in a meaningful way.
+		return x
+	case js.TypeObject:
+		if isArray(x) {
+			return createArray(x)
+		}
+		return createObject(x)
+	case js.TypeFunction:
+		var a func(...interface{}) (interface{}, error)
+		err := FromJSValue(x, &a)
+		if err != nil {
+			panic("error creating function: " + err.Error())
+		}
+		return a
+	default:
+		panic("unknown JS type: " + x.Type().String())
+	}
+}
+
+// createArray creates a slice of interface representing the js.Value.
+func createArray(x js.Value) interface{} {
+	result := make([]interface{}, x.Length())
+	for i := range result {
+		result[i] = createInterface(x.Index(i))
+	}
+	return result
+}
+
+// createObject creates a representation of the provided JS object.
+func createObject(x js.Value) interface{} {
+	// TODO: Use Object API
+	obj, err := Global().Get("Object")
+	if err != nil {
+		panic("Object not found")
+	}
+
+	var keys []string
+	err = FromJSValue(obj.Call("keys", x), &keys)
+	if err != nil {
+		panic("Object.keys returned non-string-array.")
+	}
+
+	result := make(map[string]interface{}, len(keys))
+	for _, v := range keys {
+		result[v] = createInterface(x.Get(v))
+	}
+	return result
+}
+
+// isArray calls the JS function Array.isArray to check if the provided js.Value is an array.
+func isArray(x js.Value) bool {
+	arr, err := Global().Get("Array")
+	if err != nil {
+		panic("Array not found")
+	}
+
+	return arr.Call("isArray", x).Bool()
+}
+
+// initializePointerIfNil checks if the pointer is nil and initializes it as necessary.
+func initializePointerIfNil(v reflect.Value) {
+	if v.Kind() != reflect.Ptr {
+		return
+	}
+	if v.IsNil() {
+		v.Set(reflect.New(v.Type().Elem()))
+	}
+	initializePointerIfNil(v.Elem())
 }