1  This package parses [SPDX license expression](https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60) strings describing license terms, like [package.json license strings](https://docs.npmjs.com/files/package.json#license), into consistently structured ECMAScript objects.  The npm command-line interface depends on this package, as do many automatic license-audit tools.
 2  
 3  In a nutshell:
 4  
 5  ```javascript
 6  var parse = require('spdx-expression-parse')
 7  var assert = require('assert')
 8  
 9  assert.deepEqual(
10    // Licensed under the terms of the Two-Clause BSD License.
11    parse('BSD-2-Clause'),
12    {license: 'BSD-2-Clause'}
13  )
14  
15  assert.throws(function () {
16    // An invalid SPDX license expression.
17    // Should be `Apache-2.0`.
18    parse('Apache 2')
19  })
20  
21  assert.deepEqual(
22    // Dual licensed under either:
23    // - LGPL 2.1
24    // - a combination of Three-Clause BSD and MIT
25    parse('(LGPL-2.1 OR BSD-3-Clause AND MIT)'),
26    {
27      left: {license: 'LGPL-2.1'},
28      conjunction: 'or',
29      right: {
30        left: {license: 'BSD-3-Clause'},
31        conjunction: 'and',
32        right: {license: 'MIT'}
33      }
34    }
35  )
36  ```
37  
38  The syntax comes from the [Software Package Data eXchange (SPDX)](https://spdx.org/), a standard from the [Linux Foundation](https://www.linuxfoundation.org) for shareable data about software package license terms.  SPDX aims to make sharing and auditing license data easy, especially for users of open-source software.
39  
40  The bulk of the SPDX standard describes syntax and semantics of XML metadata files.  This package implements two lightweight, plain-text components of that larger standard:
41  
42  1.  The [license list](https://spdx.org/licenses), a mapping from specific string identifiers, like `Apache-2.0`, to standard form license texts and bolt-on license exceptions.  The [spdx-license-ids](https://www.npmjs.com/package/spdx-license-ids) and [spdx-exceptions](https://www.npmjs.com/package/spdx-exceptions) packages implement the license list.  `spdx-expression-parse` depends on and `require()`s them.
43  
44      Any license identifier from the license list is a valid license expression:
45  
46      ```javascript
47      var identifiers = []
48        .concat(require('spdx-license-ids'))
49        .concat(require('spdx-license-ids/deprecated'))
50  
51      identifiers.forEach(function (id) {
52        assert.deepEqual(parse(id), {license: id})
53      })
54      ```
55  
56      So is any license identifier `WITH` a standardized license exception:
57  
58      ```javascript
59      identifiers.forEach(function (id) {
60        require('spdx-exceptions').forEach(function (e) {
61          assert.deepEqual(
62            parse(id + ' WITH ' + e),
63            {license: id, exception: e}
64          )
65        })
66      })
67      ```
68  
69  2.  The license expression language, for describing simple and complex license terms, like `MIT` for MIT-licensed and `(GPL-2.0 OR Apache-2.0)` for dual-licensing under GPL 2.0 and Apache 2.0.  `spdx-expression-parse` itself implements license expression language, exporting a parser.
70  
71      ```javascript
72      assert.deepEqual(
73        // Licensed under a combination of:
74        // - the MIT License AND
75        // - a combination of:
76        //   - LGPL 2.1 (or a later version) AND
77        //   - Three-Clause BSD
78        parse('(MIT AND (LGPL-2.1+ AND BSD-3-Clause))'),
79        {
80          left: {license: 'MIT'},
81          conjunction: 'and',
82          right: {
83            left: {license: 'LGPL-2.1', plus: true},
84            conjunction: 'and',
85            right: {license: 'BSD-3-Clause'}
86          }
87        }
88      )
89      ```
90  
91  The Linux Foundation and its contributors license the SPDX standard under the terms of [the Creative Commons Attribution License 3.0 Unported (SPDX: "CC-BY-3.0")](http://spdx.org/licenses/CC-BY-3.0).  "SPDX" is a United States federally registered trademark of the Linux Foundation.  The authors of this package license their work under the terms of the MIT License.