bcx-expression-evaluator

Safely evaluate a JavaScript-like expression in given context.

In Buttonwood, we heavily use meta-data (JSON format) to deliver business logic from backend to front-end. We don't want to design a meta-data format too complex to maintain, this tool allows us to define some light logic in pure string, way more flexible than rigid meta-data, much safer and more maintainable than passing js function as string (we did that) from backend to front-end.

This tool was mainly extracted, modified and extended from the expression parser of aurelia-binding.

Install

npm install bcx-expression-evaluator

Document

function evaluate(expression, context, helper, opts)

• expression: the expression string to be evaluated
• context: the input model object
• helper: optional helper object
• opts: optional hashmap, currently only support rejectAssignment and stringInterpolationMode
  • rejectAssignment rejects assignment in expression
  • stringInterpolationMode treats the whole expression like if it's in backticks `expression`

function evaluateStringInterpolation is a short-cut to call evaluate with stringInterpolationMode option.

Usage (in es6 syntax)

import {evaluate, evaluateStringInterpolation} from 'bcx-expression-evaluator';

const context = {
    a: 1,
    b: 2,
    c: {
    one: 'one',
    two: 'two'
    },
    avg: function() { return (this.a + this.b) / 2; }
};

evaluate('avg() > a ? c.one : c.two', context);  // => 'one';

use some helper

const helper = {
    limit: 5,
    sum: (v1, v2) => v1 + v2
};

evaluate('sum(a, b) > limit', context, helper);  // => false;

access context object itself with special $this variable

evaluate('$this', context); // => the context object
evaluate('$this.a', context); // => 1

explicitly access helper object with special $parent variable

(carried over from aurelia-binding, might change $parent to $helper in future releases.)

evaluate('a', {a:1}, {a:2}); // => 1
evaluate('$this.a', {a:1}, {a:2}); // => 1
evaluate('$parent.a', {a:1}, {a:2}); // => 2

support es6 string interpolation

evaluate('`${a+1}`', {a:1}); // => '2'

You can evaluate a string interpolation without backtick "`"

evaluate('${a+1}', {a:1}, null, {stringInterpolationMode: true}); // => '2'
evaluateStringInterpolation('${a+1}', {a:1}); // => '2'

You don't have to escape backtick in stringInterpolationMode

evaluate('`\\`${a+1}\\``', {a:1}); // => '`2`', beware you need double escape as we are writing expression in string quotes
evaluate('`${a+1}`', {a:1}, null, {stringInterpolationMode: true}); // => '`2`'
evaluateStringInterpolation('`${a+1}`', {a:1}); // => '`2`'

safe. It is not an eval in JavaScript, doesn't have access to global JavaScript objects

evaluate('parseInt(a, 10)', {a:"7"}) // => undefined

// only have access to context and helper
evaluate('parseInt(a, 10)', {a:"7"}, {parseInt: parseInt}) // => 7

silent most of the time

evaluate('a.b', {}) // => undefined, no error thrown
evaluate('a.b || c', {c: 'lorem'}) // => 'lorem', no error thrown

you can use assignment to mutate context object (or even helper object)

let obj = {a: 1, b: 2};
evaluate('a = 3', obj); // obj is now {a: 3, b: 2}
evaluate('b > 3 ? (a = true) : (a = false)', obj); // obj is now {a: false, b: 2}

disable assignment if you don't need it

This doesn't eliminate side effect, it would not prevent any function you called in bcx-expression to mutate something.

evaluate('a=1', {a:0}, null, {rejectAssignment: true}); // throws error

Difference from real JavaScript expression

bcx-expression looks like JavaScript expression, but there are some difference.

wrong reference results undefined instead of error

let obj = {a: 1};
obj.b.a // => error
evaluate('b.a', obj); // => undefined

default result for +/- operators

Behaviour carried over from aurelia-binding.

// +/- behaviour in normal JavaScript expression
undefined + 1 // => NaN
1 + undefined // => NaN
null + 1 // => 1
1 + null // => 1
undefined + undefined // => NaN
null + null // => 0

// in bcx-expression, + and - ignores undefined/null value,
// if both left and right parts are (evaluated to) undefined/null, result default to 0
evaluate('undefined + 1'); // => 1
evaluate('1 + undefined'); // => 1
evaluate('null + 1'); // => 1
evaluate('1 + null'); // => 1
evaluate('undefined + undefined'); // => 0
evaluate('null + null'); // => 0

no function expression

// all following JavaScript expressions would not work in bcx-expression
(function(){return 1})()
(() => 1)()
arr.sort((a, b) => a > b)

// but this would work
arr.sort(aHelperFunc)

no regular expression support

Regex syntax is too complex to be supported for our AST (abstract syntax tree).

// regex literal would not work in bcx-expression.
/\w/.test(string)

One way to bypass this is to supply regex literal in helper object.

evaluate('tester.test(str)', {str: '%'}, {tester: /\w/});

some JavaScript operators would not work

typeof, instanceof, delete would not work, because bcx-expression is not real JavaScript.

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