Research on the use of ChatGPT in education has proliferated in the past years. However, there is limited understanding of how English as a Foreign Language (EFL) learners engage emotionally with and utilize ChatGPT during the pre-writing stage. Furthermore, how learners' affective engagement (emotional responses) with this tool in the planning phase may be correlated with their subsequent writing remains unclear. This study aims to address these gaps by adopting a mixed-methods approach involving 56 first-year university students who completed two argumentative writing tasks, one with and one without ChatGPT support. Data were collected through multiple sources encompassing written texts, questionnaires, ChatGPT logs, note-taking sheets, and semi-structured interviews. Results showed that learners used a broader range of strategies in the prewriting stage and produced better text quality in the subsequent stage in the ChatGPT condition than in the non-ChatGPT mode. Moreover, while learners exhibited positive affective engagement in both conditions, those with ChatGPT assistance demonstrated considerably higher levels of emotional responses. Additionally, a moderate positive correlation was observed between learners' affective engagement with ChatGPT in the pre-writing phase and their overall writing performance. Pedagogically, it is recommended to integrate ChatGPT, with proper training, in the pre-writing stage to enhance both emotional positivity and text quality. Nonetheless, these benefits appear to be short-term, and further research is needed to explore the long-term impact of ChatGPT on the planning and other stages of EFL writing.