#  Type guarding is supported with ``instanceof`` and ``as``

Rule ``arkts-no-is``

**Severity: error**

ArkTS does not support the ``is`` operator, which must be replaced by the
``instanceof`` operator. Note that the fields of an object must be cast to the
appropriate type with the ``as`` operator before use.


## TypeScript


```

    class Foo {
        foo: number = 0
        common: string = ""
    }

    class Bar {
        bar: number = 0
        common: string = ""
    }

    function isFoo(arg: any): arg is Foo {
        return arg.foo !== undefined
    }

    function doStuff(arg: Foo | Bar) {
        if (isFoo(arg)) {
            console.log(arg.foo)    // OK
            console.log(arg.bar)    // Compile-time error
        } else {
            console.log(arg.foo)    // Compile-time error
            console.log(arg.bar)    // OK
        }
    }

    doStuff({ foo: 123, common: '123' })
    doStuff({ bar: 123, common: '123' })

```

## ArkTS


```

    class Foo {
        foo: number = 0
        common: string = ""
    }

    class Bar {
        bar: number = 0
        common: string = ""
    }

    function isFoo(arg: Object): boolean {
        return arg instanceof Foo
    }

    function doStuff(arg: Object): void {
        if (isFoo(arg)) {
            let fooArg = arg as Foo
            console.log(fooArg.foo)     // OK
            console.log(arg.bar)        // Compile-time error
        } else {
            let barArg = arg as Bar
            console.log(arg.foo)        // Compile-time error
            console.log(barArg.bar)     // OK
        }
    }

    function main(): void {
        doStuff(new Foo())
        doStuff(new Bar())
    }

```