eth-expression-parser v0.0.10

eth-expression-parser

Expression parser for Ethereum expressions.

This library allows you to evaluate expressions or conditions that may contain:

• Mathematical operators: +, -, *, /
• Comparisons operators: <, >, <=, =>, ==, !=
• Boolean oparator: &&, ||, !
• Parenthesis: ()
• Ether denominations: ether, gwei, wei
• Single quoted strings: 'string'
• Numbers: 20, 0xdead, 1e18
• Booleans: true, false

It also has support for:

• Indexing: array[3]
• Member access: object.property, obj.args
• Callable objects/functions: foo(), sum(40, 2), object.func(), obj.print(1, 2, 3, 'text')
• Variable names: myVariable, _var

Installation

npm install eth-expression-parser

Usage

To evaluate an expression you should provide the expression as well as a context that contains the value of all the variables contained in the expression.

const { evaluate } = require("eth-expression-parser");
const ethers = require("ethers");

const expr = "(1e18 == 0x1 ether) && (obj.sum(a, 2, d[1][0]) + 4 > 3 wei) && (b[2] == 'x')";

const context = {
    a: ethers.BigNumber.from(1),
    b: "text",
    d: [9, [3]],
    obj: {
        sum(a, b, c) {
            return a.add(b).add(c);
        },
    }
};

console.log(evaluate(expr, context)); // true

If you are familiar with nearley, you can get the parser or the grammar and use them however you prefer.

const parser = require("eth-expression-parser");

const grammar = parser.getNearleyGrammar();
const _parser = parser.getNearleyParser();

You can also get the function to evaluate your expression if needed.

const { parse, toBigNumber } = require("eth-expression-parser");

const expr = "a < b";

const evaluate = parse(expr);

console.log(evaluate({a: toBigNumber(1), b: toBigNumber(3)})); // true
console.log(evaluate({a: "c", b: "a"})); // false

Underlying Types

The types used for booleans and string are the javascript native ones, but in case of numbers the type used is ethers.BigNumber.

The usage of ethers.BigNumber as underlying type implies that all the numerical values used in the context must also be of type ethers.BigNumber in order to be able to recognize them correctly.

You can also use any method of the underlying types due to the function calls support:

const { evaluate } = require("eth-expression-parser");

const expr = "1.toHexString() == '0x01'";

console.log(evaluate(expr, {})); // true

Extra notes:

• There is not support for floating precision numbers.