1# Type guarding is supported with ``instanceof`` and ``as`` 2 3Rule ``arkts-no-is`` 4 5**Severity: error** 6 7ArkTS does not support the ``is`` operator, which must be replaced by the 8``instanceof`` operator. Note that the fields of an object must be cast to the 9appropriate type with the ``as`` operator before use. 10 11 12## TypeScript 13 14 15``` 16 17 class Foo { 18 foo: number = 0 19 common: string = "" 20 } 21 22 class Bar { 23 bar: number = 0 24 common: string = "" 25 } 26 27 function isFoo(arg: any): arg is Foo { 28 return arg.foo !== undefined 29 } 30 31 function doStuff(arg: Foo | Bar) { 32 if (isFoo(arg)) { 33 console.log(arg.foo) // OK 34 console.log(arg.bar) // Compile-time error 35 } else { 36 console.log(arg.foo) // Compile-time error 37 console.log(arg.bar) // OK 38 } 39 } 40 41 doStuff({ foo: 123, common: '123' }) 42 doStuff({ bar: 123, common: '123' }) 43 44``` 45 46## ArkTS 47 48 49``` 50 51 class Foo { 52 foo: number = 0 53 common: string = "" 54 } 55 56 class Bar { 57 bar: number = 0 58 common: string = "" 59 } 60 61 function isFoo(arg: Object): boolean { 62 return arg instanceof Foo 63 } 64 65 function doStuff(arg: Object): void { 66 if (isFoo(arg)) { 67 let fooArg = arg as Foo 68 console.log(fooArg.foo) // OK 69 console.log(arg.bar) // Compile-time error 70 } else { 71 let barArg = arg as Bar 72 console.log(arg.foo) // Compile-time error 73 console.log(barArg.bar) // OK 74 } 75 } 76 77 function main(): void { 78 doStuff(new Foo()) 79 doStuff(new Bar()) 80 } 81 82``` 83 84 85