How to get the Environment Variables in our ReactTS App

In the Pages Dashboard → Settings → Environment Variables → I’ve created a SECRET API_KEY_BACKEND. Now, I want to retrieve that SECRET in React (I understand that Pages works as SSR, so the SECRET shouldn’t be exposed, right?)

For example: In this section in ReactTS: I want to, for example, use the API_BACKEND_USER declared in the Cloudflare Dashboard.

export const userGetTokenAuthExtraReducer = createAsyncThunk(
    "/token",
    async (action, thunkAPI) => {
        try {
            const credentials =
                userApiKeys.API_BACKEND_USER + ":" + userApiKeys.API_BACKEND_PASSWORD;
            const encodedCredentials = encode(credentials);
            const response = await axios.post(
                userApiKeys.API_DOMAIN + "/token",
                {},
                {
                    headers: {
                        Authorization: "Basic " + encodedCredentials,
                    },
                }
            );
            userApiKeys.API_BEARER_TOKEN = response.data
            return response.data;
        } catch (error) {
            console.log("userSlice -> userGetTokenAuth -> ERROR response", error);
            return thunkAPI.rejectWithValue(error);
        }
    }
);