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Traffic Light Solution

Senior Front End Engineer, Ofair


In AppComponent, we define color configurations using the commonly used TypeScript Record type, Record<Colors, ColorConfig<Colors>> and pass that configuration into TrafficLightComponent.

In TrafficLightComponent, during the OnInit hook, we set currentColor to the passed colors and invoke the updateLight method. During the update, we perpetually run setTimeout in a recursive manner. Upon entering the method, we destructure the config object and assign the duration and next variables. Then, we execute setTimeout with the provided duration and assign the next color from our config as the current color. After that, we run the updateLight() method again.


The rendering of this component is pretty straightforward and can be achieved with Flexbox. With Flexbox, it's also easy to change the layout of the lights from a vertical one to a horizontal one just by changing the flex-direction property.


type Layout = 'vertical' | 'horizontal';
type DefaultColors = 'green' | 'yellow' | 'red';

Note: Take a look how Colors is related here and in next field.

type ColorConfig<Colors> = {
backgroundColor: string;
duration: number;
order: number;
next: Colors;

Main config type for traffic lights that requires Record type (something like Map in JavaScript) where key has to be Colors and value ColorConfig<Colors> from previous type.

export type TrafficLightConfig<
Colors extends DefaultColors,
> = Record<Colors, ColorConfig<Colors>>;

Angular Insights

  • You can adopt a more advanced, reactive-like approach using RxJS (without using subscriptions as possible). However, if you opt for subscriptions, remember about destroying them to prevent memory leaks.

  • You might create service for maintaining the state. Inject the service in smart component and crete dumb components that only display and interact with the UI and all events from dumb components are passed to the smart one.

  • If you're confident with the latest Angular versions, consider using signals standalone API.

  • You can try to focus more on keywords such as readonly, private, public and void.

  • If you're creating a bigger application it would be good practice to use styles per component instead of putting all styles in one file.

Test Cases

  • Observe that each light show up for the specified duration.
  • Observe that the lights transition to the next state correctly after the specified duration.


For a11y reasons, we add an aria-label to the component to indicate the current light and aria-live="polite" to announce the current active light. The contents of the component (the lights) are for visual purposes and aren't important to screen readers, they can be hidden with aria-hidden="true".