How to reject a promise from inside then function

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Am I just supposed to add return Promise.reject(validationError);?

Yes. However, it’s that complicated only in jQuery, with a Promise/A+-compliant library you also could simply

throw validationError;

So your code would then look like

someActionThatReturnsAPromise()
    .then(modifyResource)
    .then(function(modifiedResource) {
        if (!isValid(modifiedResource))
            throw getValidationError(modifiedResource);
        // else !
        return modifiedResource;
    })
    .catch(function() {
        // oh noes
    });

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