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Usage

Requires Node.js version 14 or greater.

Evaluates an estree expression from [@babel/parser][], [esprima][], [acorn][], or any other library that parses and returns a valid estree expression.

const { evaluate } = require('{%= name %}');

evaluate(expressionTree[, context]); // async
evaluate.sync(expressionTree[, context]); // sync

See the unit tests for hundreds of additional usage examples.

Params

The evaluate function takes the following arguments:

  • expressionTree {object} - a valid estree expression AST.
  • context {object} - a data object with values to replace variables in expressions

Usage with Babel

Most of the examples in this document assume the following setup code is used:

const { evaluate } = require('{%= name %}');
const { parseExpression } = require('@babel/parser');

// parse your JavaScript expression
const ast = parseExpression('1 + 2);

// evaluate synchronously
console.log(evaluate.sync(ast)); //=> 3

// or asynchronously
console.log(await evaluate(ast)); //=> 3

Usage with Esprima

[Esprima][esprimar] doesn't have a "parseExpression" method like @babel/parser, so you'll need to return the expression from the AST, like so:

const { parse } = require('esprima');
const { evaluate } = require('{%= name %}');
const ast = parse('[1, 2, 3].map(n => n * x);').body[0].expression;

// evaluate synchronously
console.log(evaluate.sync(ast)); // =>, [2, 4, 6]

// or asynchronously
console.log(await evaluate(ast)); // =>, [2, 4, 6]

API

.evaluate

Evaluate expresssions asynchronously.

console.log(await evaluate(parse('1 + 2'))); //=> 3
console.log(await evaluate(parse('5 * 2'))); //=> 10
console.log(await evaluate(parse('1 > 2'))); //=> false
console.log(await evaluate(parse('1 < 2'))); //=> true

// with context object
console.log(await evaluate(parse('page.title === "home"'), { page: { title: 'home' } })); //=> true

.evaluate.sync

Evaluate expresssions synchronously.

console.log(evaluate.sync(parse('1 + 2'))); //=> 3
console.log(evaluate.sync(parse('5 * 2'))); //=> 10
console.log(evaluate.sync(parse('1 > 2'))); //=> false
console.log(evaluate.sync(parse('1 < 2'))); //=> true

// with context object
console.log(evaluate.sync(parse('page.title === "home"'), { page: { title: 'home' } })); //=> true

.variables

Get an array of variables from an expression:

const { parseExpression } = require('@babel/parser');
const { variables } = require('{%= name %}');

console.log(variables(parseExpression('x * (y * 3) + z.y.x'))); //=> ['x', 'y', 'z']
console.log(variables(parseExpression('(a || b) ? c + d : e * f'))); //=> ['a', 'b', 'c', 'd', 'e', 'f']

Options

booleanLogicalOperators

Type: boolean Default: undefined

Force logical operators to return a boolean result.

console.log(await evaluate(parse('a && b'), { a: undefined, b: true })); //=> undefined
console.log(await evaluate(parse('a && b'), { a: undefined, b: false })); //=> undefined
console.log(await evaluate(parse('a || b'), { a: false, b: null })); //=> null
console.log(await evaluate(parse('a || b'), { a: false, b: undefined })); //=> undefined

//
// With booleanLogicalOperators enabled
//

const options = {
  booleanLogicalOperators: true
};

console.log(await evaluate(parse('a || b'), { a: false, b: null }, options)); //=> false
console.log(await evaluate(parse('a && b'), { a: undefined, b: true }, options)); //=> false
console.log(await evaluate(parse('a && b'), { a: undefined, b: false }, options)); //=> false
console.log(await evaluate(parse('a || b'), { a: false, b: undefined }, options)); //=> false

functions

Type: boolean Default: false

Allow function calls to be evaluated. This is unsafe, please enable this option at your own risk.

Example

const { parse } = require('esprima');
const { generate } = require('escodegen');
const { evaluate } = require('{%= name %}');

const options = {
  functions: true
};

// works with native methods
console.log(evaluate.sync(parse('/([a-z]+)/.exec(" foo ")'), { x: 2 }, options));
//=> [ 'foo', 'foo', index: 1, input: ' foo ', groups: undefined ]

// and functions defined on the context
console.log(evaluate.sync('a.upper("b")', { a: { upper: v => v.toUpperCase() } }, options);
//=> 'B'

However, this does NOT support function expressions or function statements. To enable function statements and expressions (not just function calls) to be evaluated, you must also use the generate option.

generate

Type: boolean Default: undefined

Enable support for function statements and expressions by enabling the functions option AND by passing the .generate() function from the [escodegen][] library.

Example

const escodegen = require('escodegen');
const { parse } = require('esprima');
const { evaluate } = require('{%= name %}');

const options = {
  functions: true,
  generate: escodegen.generate
};

console.log(await evaluate(parse('[1, 2, 3].map(n => n * x);'), { x: 2 }, options))); // =>, [2, 4, 6]

regexOperator

Type: boolean Default: true

Enable the =~ regex operator to support testing values without using functions (example name =~ /^a.*c$/).

Why is this needed?

In expressions, if you wish to test a value using a regular expression, you have two options:

  1. Enable function support so that you can use methods like .test() and .match(), or
  2. Use this option, which uses a special syntax to match against regular expressions without evaluating an functions.

In other words, instead of having to do this:

console.log(evaluate.sync(parse('/^ab+c$/ig.test("abbbbbc")'), {}, { functions: true }));

You can do this:

console.log(evaluate.sync(parse('name =~ /^a.*c$/'), { name: 'abc' }));
console.log(evaluate.sync(parse('name =~ regex'), { name: 'abc', regex: /^a.*c$/ }));

strict

Type: boolean Default: false

Throw an error when variables are undefined.

withMembers

Type: boolean Default: undefined

Used with the variables method to return nested variables (e.g. variables with dot notation, like foo.bar.baz).

Examples

Operators

Supports all JavaScript operators with the exception of assignment operators (=, +=, etc):

// Arithmetic operators
evaluate('a + b');
evaluate('a - b');
evaluate('a / b');
evaluate('a * b');
evaluate('a % b');
evaluate('a ** b');

// Relational operators
evaluate('a instanceof b');
evaluate('a < b');
evaluate('a > b');
evaluate('a <= b');
evaluate('a >= b');

// Equality operators
evaluate('a !== b');
evaluate('a === b');
evaluate('a != b');
evaluate('a == b');

// Bitwise shift operators
evaluate('a << b');
evaluate('a >> b');
evaluate('a >>> b');

// Binary bitwise operators
evaluate('a & b');
evaluate('a | b');
evaluate('a ^ b');

// Binary logical operators
evaluate('a && b'); // Logical AND.
evaluate('a || b'); // Logical OR.
evaluate('a ?? b'); // Nullish Coalescing Operator.