Usage Contexts

The expression engine is used in four different features, each with slightly different behavior, available variables, and capabilities.

Find & Replace

Used in: Editor → Find & Replace → Replace field (when replacement contains {{...}})

How it works: When you enter a replacement value like {{old.toUpperCase()}}, the expression is evaluated for each matched value. The old variable holds the current value being replaced.

Available variables:

• old — the current value being replaced (auto-wrapped in StringWrapper for method access)

Example:

Replace modes:

• Value mode — replaces only matched values
• Key mode — replaces matched key names
• Set mode — sets matched fields to a new value

Error behavior: On error, the replacement value becomes [ERROR: message] as a string.

Tabular Operations

Used in: Pipeline → Tabular Operations utility

How it works: Expressions are evaluated per row in an array of objects. Each row’s fields are directly accessible as variables.

Available variables:

• All fields of the current row (e.g., name, age, price)
• Built-in functions (Math, Date, stringify, toString, keys, values)

Operations that use expressions

filter — Keep rows matching a condition

condition: {{age >= 18}}

The expression is evaluated for each row. Rows where the result is truthy are kept.

// Input
[
  { "name": "Alice", "age": 25 },
  { "name": "Bob", "age": 12 },
  { "name": "Carol", "age": 30 }
]
 
// After filter with {{age >= 18}}
[
  { "name": "Alice", "age": 25 },
  { "name": "Carol", "age": 30 }
]

addColumn — Compute a new field

name:    fullName
formula: {{firstName + " " + lastName}}

The expression is evaluated for each row, and the result is stored as a new field.

// Input
[{ "firstName": "Alice", "lastName": "Smith" }]
 
// After addColumn "fullName" with {{firstName + " " + lastName}}
[{ "firstName": "Alice", "lastName": "Smith", "fullName": "Alice Smith" }]

deleteWhere — Remove rows matching a condition

where: {{status == "inactive"}}

Inverse of filter — rows where the condition is truthy are removed.

updateWhere — Update matching rows

where: {{age < 18}}
set:   { "category": "minor" }

Rows matching the condition have their fields updated. The set values can also be expressions:

set: { "displayName": "{{firstName + \" \" + lastName}}" }

Generate CSV

Used in: Pipeline → JSON to CSV utility (with column mappings)

How it works: When generating CSV with custom column mappings, each mapping can contain an expression that is evaluated per row.

Available variables:

• All fields of the current row

Example mapping:

columns:
  - header: "Full Name"
    expression: "{{firstName}} {{lastName}}"
  - header: "Address"
    expression: "{{stringify(address)}}"
  - header: "Active"
    expression: "{{status == \"active\" ? \"Yes\" : \"No\"}}"

Error behavior: On error, the CSV cell contains the error message string.

Compute Fields

Used in: Pipeline → Compute Fields utility

How it works: The {{fieldName}} placeholders are replaced with field values via simple string substitution. Then the result is auto-detected as one of three types:

1. Conditional: {{field}} >= value ? "trueResult" : "falseResult"
2. Math: {{field1}} * {{field2}} (when all values are numbers)
3. String template: {{firstName}} {{lastName}} (default)

What works:

{{firstName}} {{lastName}}                    → string concatenation
{{price}} * {{quantity}}                      → math (auto-detected)
{{age}} >= 18 ? "adult" : "minor"            → conditional (auto-detected)

What does NOT work in Compute (but works elsewhere):

The key difference: Compute resolves {{field}} to the raw value first, then tries to evaluate the resulting string. The full expression engine parses the expression as an AST and evaluates it with access to all functions and operators.

Comparison table