How to use the esp-js.utils.isFunction function in esp-js

private _getRenderModel(model: any): TModel {
        // does the given model have a decorated function we can invoke to get a different model to render?
        if (EspDecoratorUtil.hasMetadata(model)) {
            let metadata: GetEspReactRenderModelMetadata = EspDecoratorUtil.getCustomData(model, GetEspReactRenderModelConsts.CustomDataKey);
            if (metadata) {
                return model[metadata.functionName]();
            }
        }
        // else see if there is a function with name GetEspReactRenderModelConsts.HandlerFunctionName we can invoke to get a different model to render?
        let renderModelGetter = model[GetEspReactRenderModelConsts.HandlerFunctionName];
        if (renderModelGetter && utils.isFunction(renderModelGetter)) {
            return renderModelGetter.call(model);
        }
        // else just return the default model
        return model;
    }
}