Writing expressions

Expressions are used in a number of ways for configuring your functions. They are used for:

All expressions are defined using the Common Expression Language (CEL). CEL offers simple, fast, non-turing complete expressions. It allows Inngest to evaluate millions of expressions for all users at scale.

Types of Expressions

Within the scope of Inngest, expressions should evaluate to either a boolean or a value:

  • Booleans - Any expression used for conditional matching should return a boolean value. These are used in wait for event, cancellation, and the function trigger's if option.
  • Values - Other expressions can return any value which might be used as keys (for example, concurrency, rate limit, debounce or idempotency keys) or a dynamic value (for example, run priority).


  • event refers to the event that triggered the function run, in every case.
  • async refers to a new event in step.waitForEvent and cancellation. It's the incoming event which is matched asynchronously. This is only present when matching new events in a function run.


Most expressions are given the event payload object as the input. Expressions that match additional events (for example, wait for event, cancellation) will also have the async object for the matched event payload. To learn more, consult this reference of all the operators available in CEL.

Boolean Expressions

// Match a field to a string
"event.data.billingPlan == 'enterprise'"

// Number comparison
"event.data.amount > 1000"

// Combining multiple conditions
"event.data.billingPlan == 'enterprise' && event.data.amount > 1000"
"event.data.billingPlan != 'pro' || event.data.amount < 300"

// Compare the function trigger with an inbound event (for wait for event or cancellation)
"event.data.userId == async.data.userId"

// Alternatively, you can use JavaScript string interpolation for wait for event
`${userId} == async.data.userId` // => "user_1234 == async.data.userId"

// Advanced CEL methods (see reference linked above):
// Check if a string contains a substring

// Check that a field is set

// Compare timestamps
"timestamp(event.data.created) > timestamp('2024-01-01T00:00:00Z')"
"timestamp(event.data.createdAt) + duration('5m') > timestamp(event.data.expireAt)"

Value Expressions


// Use the user's id as a concurrency key
"event.data.id" // => "1234"

// Concatenate two strings together to create a unique key
`event.data.userId + "-" + event.type` // => "user_1234-signup"

// Advanced CEL methods (see reference linked above):
// Convert a number to a string for concatenation
`string(event.data.amount) + "-" event.data.planId`

Dynamic Values

// Return a 0 priority if the billing plan is enterprise, otherwise return 1800
`event.data.billingPlan == 'enterprise' ? 0 : 1800`

// Return a value based on multiple conditions
`event.data.billingPlan == 'enterprise' && event.data.requestNumber < 10 ? 0 : 1800`

// Advanced CEL methods (see reference linked above):
// Return a priority if the value is set in the payload
`has(event.data.priority) ? event.data.priority : 0`


  • Use + to concatenate strings
  • Use == for equality checks
  • You can use single ' or double quotes " for strings, but we recommend sticking with one for code consistency
  • When working with the TypeScript SDK, write expressions within backticks ` to use quotes in your expression or use JavaScript's string interpolation.
  • Use ternary operators to return default values
  • When using the or operator (||), CEL will always return a boolean. This is different from JavaScript, where the or operator returns the value of the statement left of the operator if truthy. Use the ternary operator (?) instead of || for conditional returns.

CEL Helpers & Macros

This is a non-exhaustive list of CEL helpers and macros that are useful for writing expressions:

// Check if a field is set

// Convert a number to a string

// Convert a string to an int

// Get the first item in an array

// Check if an item exists in an array
"event.data.items.exists(e, e == 'shirt_1234')"

// Check if only one item matches a condition
"event.data.items.exists_one(e, e.starsWith('shirt_'))"

// Check if all items in an array match a condition
"event.data.amounts.all(n, n > 10)"

// Convert a timestamp to an int (unix timestamp)

// Add a duration to a timestamp
"timestamp(event.data.createdAt) + duration('5m')"

Testing out expressions

You can test out expressions on Undistro's CEL Playground. It's a great way to quickly test out more complex expressions, especially with conditional returns.