| 1 | --- |
| 2 | title: Defer Await Until Needed |
| 3 | impact: HIGH |
| 4 | impactDescription: avoids blocking unused code paths |
| 5 | tags: async, await, conditional, optimization |
| 6 | --- |
| 7 | |
| 8 | ## Defer Await Until Needed |
| 9 | |
| 10 | Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them. |
| 11 | |
| 12 | **Incorrect (blocks both branches):** |
| 13 | |
| 14 | ```typescript |
| 15 | async function handleRequest(userId: string, skipProcessing: boolean) { |
| 16 | const userData = await fetchUserData(userId) |
| 17 | |
| 18 | if (skipProcessing) { |
| 19 | // Returns immediately but still waited for userData |
| 20 | return { skipped: true } |
| 21 | } |
| 22 | |
| 23 | // Only this branch uses userData |
| 24 | return processUserData(userData) |
| 25 | } |
| 26 | ``` |
| 27 | |
| 28 | **Correct (only blocks when needed):** |
| 29 | |
| 30 | ```typescript |
| 31 | async function handleRequest(userId: string, skipProcessing: boolean) { |
| 32 | if (skipProcessing) { |
| 33 | // Returns immediately without waiting |
| 34 | return { skipped: true } |
| 35 | } |
| 36 | |
| 37 | // Fetch only when needed |
| 38 | const userData = await fetchUserData(userId) |
| 39 | return processUserData(userData) |
| 40 | } |
| 41 | ``` |
| 42 | |
| 43 | **Another example (early return optimization):** |
| 44 | |
| 45 | ```typescript |
| 46 | // Incorrect: always fetches permissions |
| 47 | async function updateResource(resourceId: string, userId: string) { |
| 48 | const permissions = await fetchPermissions(userId) |
| 49 | const resource = await getResource(resourceId) |
| 50 | |
| 51 | if (!resource) { |
| 52 | return { error: 'Not found' } |
| 53 | } |
| 54 | |
| 55 | if (!permissions.canEdit) { |
| 56 | return { error: 'Forbidden' } |
| 57 | } |
| 58 | |
| 59 | return await updateResourceData(resource, permissions) |
| 60 | } |
| 61 | |
| 62 | // Correct: fetches only when needed |
| 63 | async function updateResource(resourceId: string, userId: string) { |
| 64 | const resource = await getResource(resourceId) |
| 65 | |
| 66 | if (!resource) { |
| 67 | return { error: 'Not found' } |
| 68 | } |
| 69 | |
| 70 | const permissions = await fetchPermissions(userId) |
| 71 | |
| 72 | if (!permissions.canEdit) { |
| 73 | return { error: 'Forbidden' } |
| 74 | } |
| 75 | |
| 76 | return await updateResourceData(resource, permissions) |
| 77 | } |
| 78 | ``` |
| 79 | |
| 80 | This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive. |
| 81 | |
| 82 | For `await getFlag()` combined with a cheap synchronous guard (`flag && someCondition`), see [Check Cheap Conditions Before Async Flags](./async-cheap-condition-before-await.md). |
| 83 |