Number that is not zero in TypeScript

Trying to find a solution in TypeScript for defining a type that represents a non-zero number:

type Task = { id: number };
const task: Task = { id: 5 };
const tasks: { [taskId: number]: Task } = { 5: task };

function getTask(taskId: number | undefined): Task | undefined {
    return taskId && tasks[taskId];
}

Playground available here

In the provided code, TypeScript raises an issue because taskId is considered a number and may potentially be 0, which is falsey. This could result in the function returning 0 instead of a Task or undefined. Realistically, it is not feasible for a task id to be 0; it will always be a positive integer. Is there a method to define the Task type in such a way that indicates to TypeScript that this number will never be falsey? It would be preferable to avoid adding extra conditional statements for TypeScript acknowledgment.

typescript

Answer №1

There is no foolproof way for typescript to ensure the existence of a task for every possible taskId.

Options to consider include:

  • Employing the non-null assertion operator to indicate to typescript that you are confident the task will always be present
  • Limiting the type to the specific set of potential taskIds by defining them as an enum or similar structure
  • Verifying the presence of the task and raising an error if it is undefined

If your task list is dynamic, opting for the last solution may be preferable, for example:

function getTask(taskId: number): Task {
     const task = tasks[taskId]
     
     if(!task){ throw new Error(`task with id ${taskId} does not exist`) }

    return task
}

This approach helps in providing a helpful error message if your assumption is proven wrong.

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