# Restrictions on Using the TS Language for Generators<a name="EN-US_TOPIC_0000001110789000"></a>The TypeScript programming language has the following restrictions on generators:- Expressions can be used only in character strings \($\{expression\}\), **if** conditions, **ForEach** parameters, and component parameters.- None of these expressions can cause any application state variables \(**@State**, **@Link**, and **@Prop**\) to change. Otherwise, undefined and potentially unstable framework behavior may occur.- You can use **console.log** in the first line of the generator function body so that you can more easily track component re-rendering. Expressions in the log character strings also comply with the preceding restrictions.- The generator function cannot contain local variables.None of the above restrictions apply to anonymous function implementations of event-handling functions \(such as **onClick**\) and to the rest of the UI component description.Incorrect:```build() { let a: number = 1 // invalid: variable declaration not allowed console.log(`a: ${a}`) // invalid: console.log only allowed in first line of build Column() { Text('Hello ${this.myName.toUpperCase()}') // ok. ForEach(this.arr.reverse(), ..., ...) // invalid: Array.reverse modifies the @State array varible in place } buildSpecial() // invalid: no function calls Text(this.calcTextValue()) // this function call is ok.}```
