ael

AEL

Advanced Expression Language (AEL)

About

Advanced Expression Language (AEL) is a JavaScript library for use in the Browser and Node.js to parse/compile and execute/evaluate JavaScript-style expressions. The expressions are based on sequence, assignment, conditional, logical, bitwise, relational, arithmetical, functional, selective and literal constructs and hence can express arbitrary complex matchings and lookups. The result can be an arbitrary value, but usually is just a boolean one. AEL is primarily intended to be used for evaluating access control rules or expanding template variables.

Installation

$ npm install ael

Usage

$ cat sample.js
const AEL = require("ael")

const ael = new AEL({
    trace: (msg) => console.log(msg)
})

const data = {
    session: {
        user: {
            login: "rse",
            email: "[e-mail]"
        },
        tokens: [
            "455c3026-50cf-11eb-8d93-7085c287160d",
            "4600b07e-50cf-11eb-8d57-7085c287160d"
        ]
    },
}

const expr = `
    grant =~ /^login:(.+)$/ ? session?.user?.login =~ $1 :
    grant =~ /^email:(.+)$/ ? session?.user?.email =~ $1 :
    grant =~ /^token:(.+)$/ ? session?.tokens      >= $1 : false
`

const grants = [
    "login:^(?:rse|foo|bar)$",
    "email:^.+@engelschall\\.com$",
    "email:^.+@example\\.com$",
    "token:4600b07e-50cf-11eb-8d57-7085c287160d"
]

try {
    let granted = false
    for (const grant of grants) {
        if (ael.evaluate(expr, { ...data, grant })) {
            granted = true
            break
        }
    }
    console.log("GRANTED", granted)
}
catch (ex) {
    console.log("ERROR", ex.toString())
}

$ node sample.js
compile: +---(expression string)---------------------------------------------------------------------------------
compile: |
compile: |     grant =~ /^login:(.+)$/ ? session?.user?.login =~ $1 :
compile: |     grant =~ /^email:(.+)$/ ? session?.user?.email =~ $1 :
compile: |     grant =~ /^token:(.+)$/ ? session?.tokens      >= $1 : false
compile: +---(abstract syntax tree)------------------------------------------------------------------------------
compile: | ConditionalTernary [2,5]
compile: | ├── Relational (op: "=~") [2,5]
compile: | │   ├── Variable (id: "grant") [2,5]
compile: | │   └── LiteralRegExp (value: /^login:(.+)$/) [2,14]
compile: | ├── Relational (op: "=~") [2,31]
compile: | │   ├── Select [2,31]
compile: | │   │   ├── Variable (id: "session") [2,31]
compile: | │   │   ├── SelectItem (optional: true) [2,38]
compile: | │   │   │   └── Identifier (id: "user") [2,40]
compile: | │   │   └── SelectItem (optional: true) [2,44]
compile: | │   │       └── Identifier (id: "login") [2,46]
compile: | │   └── Variable (id: "$1") [2,55]
compile: | └── ConditionalTernary [3,5]
compile: |     ├── Relational (op: "=~") [3,5]
compile: |     │   ├── Variable (id: "grant") [3,5]
compile: |     │   └── LiteralRegExp (value: /^email:(.+)$/) [3,14]
compile: |     ├── Relational (op: "=~") [3,31]
compile: |     │   ├── Select [3,31]
compile: |     │   │   ├── Variable (id: "session") [3,31]
compile: |     │   │   ├── SelectItem (optional: true) [3,38]
compile: |     │   │   │   └── Identifier (id: "user") [3,40]
compile: |     │   │   └── SelectItem (optional: true) [3,44]
compile: |     │   │       └── Identifier (id: "email") [3,46]
compile: |     │   └── Variable (id: "$1") [3,55]
compile: |     └── ConditionalTernary [4,5]
compile: |         ├── Relational (op: "=~") [4,5]
compile: |         │   ├── Variable (id: "grant") [4,5]
compile: |         │   └── LiteralRegExp (value: /^token:(.+)$/) [4,14]
compile: |         ├── Relational (op: ">=") [4,31]
compile: |         │   ├── Select [4,31]
compile: |         │   │   ├── Variable (id: "session") [4,31]
compile: |         │   │   └── SelectItem (optional: true) [4,38]
compile: |         │   │       └── Identifier (id: "tokens") [4,40]
compile: |         │   └── Variable (id: "$1") [4,55]
compile: |         └── LiteralValue (value: false) [4,60]
execute: +---(evaluation recursion tree)-------------------------------------------------------------------------
execute: | ConditionalTernary {
execute: |     Relational {
execute: |         Variable {
execute: |         }: "login:^(?:rse|foo|bar)$"
execute: |         LiteralRegExp {
execute: |         }: {}
execute: |     }: true
execute: |     Relational {
execute: |         Select {
execute: |             Variable {
execute: |             }: {"user":{"login":"rse","email":"rse@enge...
execute: |                 Identifier {
execute: |                 }: "user"
execute: |                 Identifier {
execute: |                 }: "login"
execute: |         }: "rse"
execute: |         Variable {
execute: |         }: "^(?:rse|foo|bar)$"
execute: |     }: true
execute: | }: true
GRANTED true

Expression Language

The following BNF-style grammar shows the supported expression language:

//  top-level
expr             ::= sequence
                   | assignment
                   | conditional
                   | logical
                   | bitwise
                   | relational
                   | arithmentical
                   | functional
                   | selective
                   | variable
                   | literal
                   | parenthesis

//  expressions
sequence         ::= expr ("," expr)+
assignment       ::= id "=" expr
conditional      ::= expr "?" expr ":" expr
                   | expr "??" expr
logical          ::= expr ("&&" | "||") expr
                   | "!" expr
bitwise          ::= expr ("&" | "^" | "|" | "<<" | ">>") expr
                   | "~" expr
relational       ::= expr ("==" | "!=" | "<=" | ">=" | "<" | ">" | "=~" | "!~") expr
arithmethical    ::= expr ("+" | "-" | "*" | "/" | "%" | "**") expr
functional       ::= expr "?."? "(" (expr ("," expr)*)? ")"
selective        ::= expr "?."? "." ud
                   | expr "?."? "[" expr "]"
variable         ::= id
literal          ::= array | object | template | string | regexp | number | value
parenthesis      ::= "(" expr ")"

//  literals
id               ::= /[a-zA-Z_][a-zA-Z0-9_-]*/
array            ::= "[" (expr ("," expr)*)? "]"
object           ::= "{" (key ":" expr ("," key ":" expr)*)? "}"
key              ::= "[" expr "]"
                   | id
template         ::= "`" ("${" expr "}" / ("\\`"|.))* "`"
string           ::= /"(\\"|.)*"/
                   | /'(\\'|.)*'/
regexp           ::= /`(\\`|.)*`/
number           ::= /[+-]?/ number-value
number-value     ::= "0b" /[01]+/
                   | "0o" /[0-7]+/
                   | "0x" /[0-9a-fA-F]+/
                   | /[0-9]*\.[0-9]+([eE][+-]?[0-9]+)?/
                   | /[0-9]+/
value            ::= "true" | "false" | "null" | "NaN" | "undefined"

Application Programming Interface (API)

The following TypeScript definition shows the supported Application Programming Interface (API):

declare module "AEL" {
    class AEL {
        /*  create AEL instance  */
        public constructor(
            options?: {
                cache?:    number,   /*  number of LRU-cached ASTs (default: 0)      */
                trace?: (            /*  optional tracing callback (default: null)   */
                    msg:   string    /*  tracing message                             */
                ) => void
            }
        )

        /*  individual step 1: compile (and cache) expression into AST  */
        compile(
            expr:          string    /*  expression string                           */
        ): any                       /*  abstract syntax tree                        */

        /*  individual step 2: execute AST  */
        execute(
            ast:           any,      /*  abstract syntax tree                        */
            vars?:         object,   /*  data variables  (read-only)  (default: {})  */
            state?:        object    /*  state variables (read-write) (default: {})  */
        ): void

        /*  all-in-one step: evaluate (compile and execute) expression  */
        evaluate(
            expr:          string,   /*  expression string                           */
            vars?:         object,   /*  data variables  (read-only)  (default: {})  */
            state?:        object    /*  state variables (read-write) (default: {})  */
        ): any
    }
    export = AEL
}

Implementation Notice

Although AEL is written in ECMAScript 2020, it is transpiled to older environments and this way runs in really all current (as of 2021) JavaScript environments, of course.

Additionally, there are two transpilation results: first, there is a compressed ael.browser.js for Browser environments. Second, there is an uncompressed ael.node.js for Node.js environments.

The Browser variant ael.browser.js has all external dependencies asty, pegjs-otf, pegjs-util, and cache-lru directly embedded. The Node.js variant ael.node.js still requires the external dependencies asty, pegjs-otf, pegjs-util, and cache-lru.

License

Copyright © 2021-2024 Dr. Ralf S. Engelschall (http://engelschall.com/)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.