ast-compare^1.13.19

§ Quick Take

import { strict as assert } from "assert";
import compare from "ast-compare";

// Find out, does an object/array/string/nested-mix is a subset or equal to another input:
assert.equal(
  compare(
    {
      a: {
        b: "d",
        c: [],
        e: "f",
        g: "h",
      },
    },
    {
      a: {
        b: "d",
        c: [],
      },
    }
  ),
  true
);

§ Examples

• program example: Compare Arrays
• program example: Compare Plain Objects
• program example: Compare Strings
• program example: opts.hungryForWhitespace
• program example: opts.verboseWhenMismatches
• program example: Throw Errors

§ Purpose

It compares data structures, especially AST.

We use it to compare two parsed HTML/CSS trees or their branches, but you can compare anything, it will recursively traverse arrays too. For example, it powers ast-delete-object which also works on AST's.

The default mode is similar to Tap  asserts t.match and the option opts.matchStrictly is similar to t.sameStrict.

§ API

compare(bigObj, smallObj, [opts])

In other words, it's a function which takes three input arguments, third one being optional (marked by square brackets).

It returns a boolean. This library will not mutate the input arguments.

§ API - Input

• If everything from smallObj matches everything within bigObj, this library returns true.
• Otherwise, if there's a mismatch or something wrong with input args, it returns false.

§ Options object

§ API - Output

Positive answer is always boolean true.

Negative answer is either:

• boolean false (default, opts.verboseWhenMismatches off) OR
• string, explaining what didn't match (opts.verboseWhenMismatches on)

§ opts.verboseWhenMismatches

Sometimes you just want a yes/no answer is something a subset or equal to something. But sometimes, the whole point of comparison is to inform the user exactly what is mismatching. In the latter cases, set opts.verboseWhenMismatches to true. When there is no match, instead of Boolean false the main function will return a string with an explanatory message.

If you use this setting, you have to anticipate Boolean true OR something else (Boolean false or string) coming out from this library.

§ Rationale

We want to check, does a deeply-nested array of objects/strings/arrays (for example, parsed AST output) is equal or is a subset of some other AST. Normally _.isMatch would do the deed but it positively matches empty arrays against any arrays what is terrible. Hence this library. Plus, this library will accept and adapt to any sources — combinations of arrays, objects and strings. That's necessary to support any parsed AST trees - HTML or CSS or whatever.

§ Differences from _.isMatch

"Partial comparisons will match empty array and empty object source values against any array or object value, respectively." — Lodash documentation 

_.isMatch  positively matches empty arrays to everything. This is bad when you are comparing parsed HTML/CSS trees. This library doesn't do this. An empty array will not be reported as equal to a non-empty array.

// in this library:
var res = compare(["a", "b", "c"], []);
// now, res === false

§ Licence

MIT 

Copyright © 2010–2020 Roy Revelt and other contributors