const isProduction: boolean = process.env.NODE_ENV === 'production';
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const prefix: string = 'Invariant failed';
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/**
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* `invariant` is used to [assert](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions) that the `condition` is [truthy](https://github.com/getify/You-Dont-Know-JS/blob/bdbe570600d4e1107d0b131787903ca1c9ec8140/up%20%26%20going/ch2.md#truthy--falsy).
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*
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* 💥 `invariant` will `throw` an `Error` if the `condition` is [falsey](https://github.com/getify/You-Dont-Know-JS/blob/bdbe570600d4e1107d0b131787903ca1c9ec8140/up%20%26%20going/ch2.md#truthy--falsy)
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*
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* 🤏 `message`s are not displayed in production environments to help keep bundles small
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*
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* @example
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*
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* ```ts
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* const value: Person | null = { name: 'Alex' };
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* invariant(value, 'Expected value to be a person');
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* // type of `value`` has been narrowed to `Person`
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* ```
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*/
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export default function invariant(
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condition: any,
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// Not providing an inline default argument for message as the result is smaller
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/**
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* Can provide a string, or a function that returns a string for cases where
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* the message takes a fair amount of effort to compute
|
*/
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message?: string | (() => string),
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): asserts condition {
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if (condition) {
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return;
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}
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// Condition not passed
|
|
// In production we strip the message but still throw
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if (isProduction) {
|
throw new Error(prefix);
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}
|
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// When not in production we allow the message to pass through
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// *This block will be removed in production builds*
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const provided: string | undefined = typeof message === 'function' ? message() : message;
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// Options:
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// 1. message provided: `${prefix}: ${provided}`
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// 2. message not provided: prefix
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const value: string = provided ? `${prefix}: ${provided}` : prefix;
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throw new Error(value);
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}
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