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241 lines
7.2 KiB
Markdown
241 lines
7.2 KiB
Markdown
Overview [](https://travis-ci.org/lydell/js-tokens)
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========
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A regex that tokenizes JavaScript.
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```js
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var jsTokens = require("js-tokens").default
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var jsString = "var foo=opts.foo;\n..."
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jsString.match(jsTokens)
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// ["var", " ", "foo", "=", "opts", ".", "foo", ";", "\n", ...]
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```
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Installation
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============
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`npm install js-tokens`
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```js
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import jsTokens from "js-tokens"
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// or:
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var jsTokens = require("js-tokens").default
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```
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Usage
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=====
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### `jsTokens` ###
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A regex with the `g` flag that matches JavaScript tokens.
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The regex _always_ matches, even invalid JavaScript and the empty string.
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The next match is always directly after the previous.
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### `var token = matchToToken(match)` ###
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```js
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import {matchToToken} from "js-tokens"
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// or:
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var matchToToken = require("js-tokens").matchToToken
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```
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Takes a `match` returned by `jsTokens.exec(string)`, and returns a `{type:
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String, value: String}` object. The following types are available:
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- string
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- comment
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- regex
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- number
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- name
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- punctuator
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- whitespace
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- invalid
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Multi-line comments and strings also have a `closed` property indicating if the
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token was closed or not (see below).
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Comments and strings both come in several flavors. To distinguish them, check if
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the token starts with `//`, `/*`, `'`, `"` or `` ` ``.
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Names are ECMAScript IdentifierNames, that is, including both identifiers and
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keywords. You may use [is-keyword-js] to tell them apart.
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Whitespace includes both line terminators and other whitespace.
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[is-keyword-js]: https://github.com/crissdev/is-keyword-js
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ECMAScript support
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==================
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The intention is to always support the latest ECMAScript version whose feature
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set has been finalized.
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If adding support for a newer version requires changes, a new version with a
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major verion bump will be released.
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Currently, ECMAScript 2018 is supported.
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Invalid code handling
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=====================
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Unterminated strings are still matched as strings. JavaScript strings cannot
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contain (unescaped) newlines, so unterminated strings simply end at the end of
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the line. Unterminated template strings can contain unescaped newlines, though,
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so they go on to the end of input.
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Unterminated multi-line comments are also still matched as comments. They
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simply go on to the end of the input.
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Unterminated regex literals are likely matched as division and whatever is
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inside the regex.
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Invalid ASCII characters have their own capturing group.
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Invalid non-ASCII characters are treated as names, to simplify the matching of
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names (except unicode spaces which are treated as whitespace). Note: See also
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the [ES2018](#es2018) section.
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Regex literals may contain invalid regex syntax. They are still matched as
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regex literals. They may also contain repeated regex flags, to keep the regex
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simple.
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Strings may contain invalid escape sequences.
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Limitations
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===========
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Tokenizing JavaScript using regexes—in fact, _one single regex_—won’t be
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perfect. But that’s not the point either.
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You may compare jsTokens with [esprima] by using `esprima-compare.js`.
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See `npm run esprima-compare`!
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[esprima]: http://esprima.org/
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### Template string interpolation ###
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Template strings are matched as single tokens, from the starting `` ` `` to the
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ending `` ` ``, including interpolations (whose tokens are not matched
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individually).
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Matching template string interpolations requires recursive balancing of `{` and
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`}`—something that JavaScript regexes cannot do. Only one level of nesting is
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supported.
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### Division and regex literals collision ###
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Consider this example:
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```js
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var g = 9.82
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var number = bar / 2/g
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var regex = / 2/g
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```
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A human can easily understand that in the `number` line we’re dealing with
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division, and in the `regex` line we’re dealing with a regex literal. How come?
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Because humans can look at the whole code to put the `/` characters in context.
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A JavaScript regex cannot. It only sees forwards. (Well, ES2018 regexes can also
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look backwards. See the [ES2018](#es2018) section).
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When the `jsTokens` regex scans throught the above, it will see the following
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at the end of both the `number` and `regex` rows:
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```js
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/ 2/g
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```
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It is then impossible to know if that is a regex literal, or part of an
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expression dealing with division.
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Here is a similar case:
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```js
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foo /= 2/g
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foo(/= 2/g)
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```
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The first line divides the `foo` variable with `2/g`. The second line calls the
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`foo` function with the regex literal `/= 2/g`. Again, since `jsTokens` only
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sees forwards, it cannot tell the two cases apart.
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There are some cases where we _can_ tell division and regex literals apart,
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though.
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First off, we have the simple cases where there’s only one slash in the line:
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```js
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var foo = 2/g
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foo /= 2
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```
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Regex literals cannot contain newlines, so the above cases are correctly
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identified as division. Things are only problematic when there are more than
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one non-comment slash in a single line.
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Secondly, not every character is a valid regex flag.
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```js
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var number = bar / 2/e
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```
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The above example is also correctly identified as division, because `e` is not a
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valid regex flag. I initially wanted to future-proof by allowing `[a-zA-Z]*`
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(any letter) as flags, but it is not worth it since it increases the amount of
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ambigous cases. So only the standard `g`, `m`, `i`, `y` and `u` flags are
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allowed. This means that the above example will be identified as division as
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long as you don’t rename the `e` variable to some permutation of `gmiyus` 1 to 6
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characters long.
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Lastly, we can look _forward_ for information.
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- If the token following what looks like a regex literal is not valid after a
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regex literal, but is valid in a division expression, then the regex literal
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is treated as division instead. For example, a flagless regex cannot be
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followed by a string, number or name, but all of those three can be the
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denominator of a division.
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- Generally, if what looks like a regex literal is followed by an operator, the
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regex literal is treated as division instead. This is because regexes are
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seldomly used with operators (such as `+`, `*`, `&&` and `==`), but division
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could likely be part of such an expression.
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Please consult the regex source and the test cases for precise information on
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when regex or division is matched (should you need to know). In short, you
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could sum it up as:
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If the end of a statement looks like a regex literal (even if it isn’t), it
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will be treated as one. Otherwise it should work as expected (if you write sane
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code).
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### ES2018 ###
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ES2018 added some nice regex improvements to the language.
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- [Unicode property escapes] should allow telling names and invalid non-ASCII
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characters apart without blowing up the regex size.
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- [Lookbehind assertions] should allow matching telling division and regex
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literals apart in more cases.
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- [Named capture groups] might simplify some things.
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These things would be nice to do, but are not critical. They probably have to
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wait until the oldest maintained Node.js LTS release supports those features.
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[Unicode property escapes]: http://2ality.com/2017/07/regexp-unicode-property-escapes.html
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[Lookbehind assertions]: http://2ality.com/2017/05/regexp-lookbehind-assertions.html
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[Named capture groups]: http://2ality.com/2017/05/regexp-named-capture-groups.html
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License
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=======
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[MIT](LICENSE).
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