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    A tiny invariant alternative. This is a fork of the tiny-invariant package on NPM. My version allows you to customise the prefix.

    What is 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 'tiny-invariant';
    invariant(truthyValue, 'This should not throw!');
    invariant(falsyValue, 'This will throw!');
    // Error('This will throw!');
    // give a prefix
    invariant(falsyValue, 'My message', 'My prefix');
    // Error('My prefix: My message');

    You can also provide a function to generate your message, for when your message is expensive to create

    import invariant from 'tiny-invariant';
    invariant(value, () => getExpensiveMessage());

    The same can also be done for the prefix.

    Why tiny-invariant?

    The library: invariant supports passing in arguments to the invariant function in a sprintf style (condition, format, a, b, c, d, e, f). It has internal logic to execute the sprintf substitutions. The sprintf logic is not removed in production builds. tiny-invariant has dropped all of the sprintf logic. tiny-invariant allows you to pass a single string message. With template literals there is really no need for a custom message formatter to be built into the library. If you need a multi part message you can just do this: invariant(condition, 'Hello, ${name} - how are you today?')

    Type narrowing

    tiny-invariant is useful for correctly narrowing types for flow and typescript

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

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

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


    # yarn
    yarn add tiny-invariant
    # npm
    npm install tiny-invariant --save


    • We have a es (EcmaScript module) build
    • We have a cjs (CommonJS) build
    • We have a umd (Universal module definition) build in case you needed it

    We expect process.env.NODE_ENV to be available at module compilation. We cache this value

    That's it!



    npm i @arnu515/tiny-invariant

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    • arnu515