simple_expressions 1.0.0

A small, dependency-free expression evaluator for Dart and Flutter.

simple_expressions

A Dart package for parsing and evaluating expressions against JSON-like model values.

This package is licensed under the ISC License.

Install

dart pub add simple_expressions

Usage

import 'package:simple_expressions/simple_expressions.dart';

final model = {
  'user': {'name': 'Ada'},
  'score': 8,
};

executeExpression(model, 'and(eq(#user.name, "Ada"), gt(#score, 5))');
// true

final expression = parseExpression('concat("Hello, ", #user.name)');
expression.evaluateValue(model);
// "Hello, Ada"

executeExpression and Expression.evaluate return the expression's truthiness. parseExpression returns a reusable Expression; call evaluateValue when you need the raw result. Missing and null values appear as null at the public API boundary.

Example

Run the package example:

dart run example/simple_expressions_example.dart

The sample is included in the published package's Example tab.

Syntax

Constants support case-insensitive Booleans, signed, decimal, and exponent-form numeric literals, single-, double-, or backtick-quoted strings, and recursive list literals in square brackets. References begin with #. For a dotted reference, the evaluator first checks for an exact dotted key and then traverses nested map values. References cannot contain empty segments or the reserved segments __proto__, constructor, and prototype; those restrictions are retained for compatibility with the TypeScript reference.

Operators are case-insensitive:

not, empty, len, lower, upper, trim, defined, eq, contains, startswith, endswith, in, gt, lt, match, variadic and/or/concat, and lazy if.

in requires a list and uses the same loose equality as eq. trim preserves null and missing values; other falsey values become empty strings before trimming. defined is false only for null and missing values. Lists may contain nested lists, model references, and operator expressions.

The evaluator uses JavaScript-style truthiness, loose equality, and value conversion where those rules can be represented by Dart values. Numeric values are evaluated as Dart double values. match uses Dart RegExp, so patterns that rely on JavaScript-specific regex syntax are not portable.

Differences from the TypeScript reference

This port follows the TypeScript reference for grammar, operators, evaluation order, and selected safety rules. It does not reproduce every JavaScript coercion quirk. Cross-language compatibility is tested separately in simple-expressions-conformance.

Behaviour	TypeScript	This port
String to number	Number("") is 0, "Infinity" is a keyword	A string converts only if it parses as a number; "" and " " are NaN
eq on two lists or maps	Reference equality, so eq(#a, #b) on equal-looking lists is false	Structural equality: same length or key set, and every pair loosely equal, to a depth of 64
parseExpression	Returns (model) => value	Returns an Expression; call evaluateValue(model) for the raw value or evaluate(model) for truthiness
Class name	SimpleExpression	Expression
Missing vs null	Distinguishable in results (undefined vs null)	Both surface as null from evaluateValue, and are equal under eq
Model values	Any JavaScript object	String keys in Dart Map values resolve; list indexes are not traversed, and an unsupported runtime type is treated as an empty object

Numbers use the same formatting rule as the TypeScript reference: the shortest round-trip decimal, plain notation for magnitudes in [1e-6, 1e21), and lowercase exponent notation outside that range.

Model Access

Models are read when an expression asks for a value. Nothing is copied or converted up front, so an expression only visits the parts of a model that it references.

Model values can be Dart maps and lists, along with strings, numbers, booleans, and null. Map keys are matched as strings. A non-map model root resolves every reference to a missing value.

Keep these details in mind:

• Model values are read live at evaluation time. Mutations made between evaluations are visible to the next evaluation.
• List indexes and list properties are not traversed through references.
• A value with an unsupported Dart runtime type is treated as an empty object.
• Missing references surface as null from evaluateValue. Internally, missing and explicit null remain distinct, but eq, defined, and empty treat them the same.

Caching

Parsed expressions and Expression instances are cached by default. Both caches use a FIFO limit of 1,000 entries. Use SimpleExpressions.setCacheLimit, clear, disableCaches, and enableCaches to control them. Disabling caches also clears existing entries.

Security

Expressions and regex patterns are trusted input. Regex evaluation can be vulnerable to catastrophic backtracking. Do not use untrusted expressions or patterns as an authorization boundary.

Development

dart pub get
dart format .
dart analyze
dart test