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edge-lexer

Generate tokens by parsing a raw edge markup file

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Edge lexer produces a list of tokens by scanning for Edge whitelisted syntax.

This module is a blend of a lexer and an AST generator, since Edge doesn't need a pure lexer that scans for each character. Edge markup is written within other markup languages like HTML or Markdown and walking over each character is waste of resources.

Instead, this module starts by detecting for the Edge whitelisted syntax and then starts the lexical analysis within the detected markup.

Table of contents

Highlights

  • Just one dependency to standardize edge errors.
  • Uses only one regex statement. Tested against safe-regex for ReDOS
  • Support for multiline expressions
  • Collects line and columns for accurate stack traces
  • Detects for unclosed tags
  • Detects for unwrapped expressions and raises appropriate errors

Performance

Following measures are taken to keep the analysis performant.

  1. Only analyse markup that is detected as Edge whitelisted syntax.
  2. Only analyse tags, that are passed to the tokenizer. Which means even if the syntax for tags is whitelisted, the tokeniser will analyse them if they are used by your app.
  3. Do not analyse Javascript expression and leave that for edge-parser.
  4. Only uses one Regular expression.

Usage

import { Tokenizer } from 'edge-lexer'

const template = `Hello {{ username }}`
const tags = {
  if: {
    block: true,
    seekable: true,
  },
}

// Filename is required to add it to error messages
const options = {
  filename: 'welcome.edge',
}

const tokenizer = new Tokenizer(template, tags, options)

tokenizer.parse()
console.log(tokenizer.tokens)

Pre-processing lines

You can also pre-process lines before the tokenizer tokenizes them.

const options = {
  filename: 'welcome.edge',
  onLine: (line: string) => {
    // transform here and return new string value
    return line
  },
}

const tokenizer = new Tokenizer(template, {}, options)

Terms used

This guide makes use of the following terms to identify core pieces of the tokenizer.

Term Token Type Description
Tag tag Tags are used to define logical blocks in the template engine. For example if tag or include tag.
Escaped Tag e__tag Escaped tag, Edge will not evaluate it at runtime.
Mustache mustache Javascript expression wrapped in curly braces. {{ }}
Safe Mustache s__mustache Safe mustache, that doesn't escape the output {{{ }}}
Escaped Mustache e__mustache Mustache tag that is escaped
Escaped Safe Mustache es__mustache Safe Mustache tag that is escaped
Raw raw A raw string, which has no meaning for the template engine
NewLine newline Newline
Comment comment Edge specific comment block. This will be ripped off in the output.

Tokens

Following is the list of Nodes returned by the tokenizer.

Tag Token

{
  type: 'tag'
  filename: 'eval.edge',
  loc: {
    start: {
      line: 1,
      col: 4
    },
    end: {
      line: 1,
      col: 13
    }
  },
  properties: BlockProp,
  children: []
}

Escaped Tag Token

{
- type: 'tag',
+ type: 'e__tag',
  filename: 'eval.edge',
  loc: {
    start: {
      line: 1,
      col: 4
    },
    end: {
      line: 1,
      col: 13
    }
  },
  properties: BlockProp,
  children: []
}

Raw Token

{
  type: 'raw',
  filename: 'eval.edge',
  line: number,
  value: string
}

Comment Token

{
  type: 'comment',
  filename: 'eval.edge',
  line: number,
  value: string
}

NewLine Token

{
  type: 'newline',
  line: number
}

Mustache Token

{
  type: 'mustache',
  filename: 'eval.edge',
  loc: {
    start: {
      line: 1,
      col: 4
    },
    end: {
      line: 1,
      col: 13
    }
  },
  properties: Prop
}

Safe Mustache Token

{
- type: 'mustache',
+ type: 's__mustache',
  filename: 'eval.edge',
  loc: {
    start: {
      line: 1,
      col: 4
    },
    end: {
      line: 1,
      col: 13
    }
  },
  properties: Prop
}

Escaped Mustache Token

{
- type: 'mustache',
+ type: 'e__mustache',
  filename: 'eval.edge',
  loc: {
    start: {
      line: 1,
      col: 4
    },
    end: {
      line: 1,
      col: 13
    }
  },
  properties: Prop
}

Escaped Safe Mustache Token

{
- type: 'mustache',
+ type: 'es__mustache',
  filename: 'eval.edge',
  loc: {
    start: {
      line: 1,
      col: 4
    },
    end: {
      line: 1,
      col: 13
    }
  },
  properties: Prop
}
Key Value Description
type string The type of node determines the behavior of node
loc object loc is only present for tags and mustache tokens
line number line is not present for tags and mustache tokens
properties Prop Meta data for the node. See Properties to more info
value string If token is a raw or comment token, then value is the string in the source file
children array Array of recursive nodes. Only exists, when token is a tag

Properties

The properties Prop is used to define meta data for a given Node. Nodes like raw, comment and newline, doesn't need any metadata.

BlockProp

The block prop is used by the Block node. The only difference from the regular Prop is the addition of selfclosed attribute.

{
  name: string
  jsArg: string,
  selfclosed: boolean
}

Prop

{
  jsArg: string,
}
Key Description
jsArg The jsArg is the Javascript expression to evaluate. Whitespaces and newlines are preserved inside the jsArg
selfclosed Whether or not the tag was selfclosed during usage.

Mustache expressions

For mustache nodes props, the name is the type of mustache expressions. The lexer supports 4 mustache expressions.

mustache

{{ username }}

e__mustache (Escaped mustache)

The following expression is ignored by edge. Helpful when you want this expression to be parsed by a frontend template engine

@{{ username }}

s__mustache (Safe mustache)

The following expression output is considered HTML safe.

{{{ '<p> Hello world </p>' }}}

es__mustache (Escaped safe mustache)

@{{{ '<p> Not touched </p>' }}}

Errors

Errors raised by the lexer are always an instance of edge-error and will contain following properties.

error.message
error.line
error.col
error.filename
error.code

Example

{{-- Show username when exists --}} @if(username) {{-- Wrap inside h2 --}}
<h2>Hello {{ username }}</h2>
@endif

The output of the above text will be

[
  {
    "type": "comment",
    "filename": "eval.edge",
    "value": " Show username when exists ",
    "loc": {
      "start": {
        "line": 1,
        "col": 4
      },
      "end": {
        "line": 1,
        "col": 35
      }
    }
  },
  {
    "type": "tag",
    "filename": "eval.edge",
    "properties": {
      "name": "if",
      "jsArg": "username",
      "selfclosed": false
    },
    "loc": {
      "start": {
        "line": 2,
        "col": 4
      },
      "end": {
        "line": 2,
        "col": 13
      }
    },
    "children": [
      {
        "type": "newline",
        "filename": "eval.edge",
        "line": 2
      },
      {
        "type": "comment",
        "filename": "eval.edge",
        "value": " Wrap inside h2 ",
        "loc": {
          "start": {
            "line": 3,
            "col": 4
          },
          "end": {
            "line": 3,
            "col": 24
          }
        }
      },
      {
        "type": "newline",
        "filename": "eval.edge",
        "line": 3
      },
      {
        "type": "raw",
        "value": "<h2> Hello ",
        "filename": "eval.edge",
        "line": 4
      },
      {
        "type": "mustache",
        "filename": "eval.edge",
        "properties": {
          "jsArg": " username "
        },
        "loc": {
          "start": {
            "line": 4,
            "col": 13
          },
          "end": {
            "line": 4,
            "col": 25
          }
        }
      },
      {
        "type": "raw",
        "value": " </h2>",
        "filename": "eval.edge",
        "line": 4
      }
    ]
  }
]

Raised exceptions

Following the links to documented error codes raised by the lexer.

Maintainers

Harminder virk