invariant

An invariant function takes a value, and if the value is falsy then the invariant function will throw. If the value is truthy, then the function will not throw.

import { invariant } from '@dts-stn/invariant';

invariant(truthyValue, 'This should not throw!');

invariant(falsyValue, 'This will throw!');
// Error('Invariant violation: This will throw!');

Why @dts-stn/invariant?

Unlike tiny-invariant, which strips error messages in production, @dts-stn/invariant preserves them. This is the fundamental reason for its creation, as these messages are vital for debugging production issues.

Error Messages

@dts-stn/invariant allows you to pass a string message, or a function that returns a string message. Using a function that returns a message is helpful when your message is expensive to create.

import { invariant } from '@dts-stn/invariant';

invariant(condition, `Hello, ${name} - how are you today?`);

// Using a function is helpful when your message is expensive
invariant(value, () => getExpensiveMessage());

Type narrowing

@dts-stn/invariant is useful for correctly narrowing types for typescript

const value: Person | null = { name: 'Alex' }; // type of value == 'Person | null'
invariant(value, 'Expected value to be a person');
// type of value has been narrowed to 'Person'

API: (condition: any, message?: string | (() => string)) => void

• condition is required and can be anything
• message optional string or a function that returns a string (() => string)