index.js

var util = require('util')
var isProperty = require('is-property')

var INDENT_START = /[\{\[]/
var INDENT_END = /[\}\]]/

// from https://mathiasbynens.be/notes/reserved-keywords
var RESERVED = [
  'do',
  'if',
  'in',
  'for',
  'let',
  'new',
  'try',
  'var',
  'case',
  'else',
  'enum',
  'eval',
  'null',
  'this',
  'true',
  'void',
  'with',
  'await',
  'break',
  'catch',
  'class',
  'const',
  'false',
  'super',
  'throw',
  'while',
  'yield',
  'delete',
  'export',
  'import',
  'public',
  'return',
  'static',
  'switch',
  'typeof',
  'default',
  'extends',
  'finally',
  'package',
  'private',
  'continue',
  'debugger',
  'function',
  'arguments',
  'interface',
  'protected',
  'implements',
  'instanceof',
  'NaN',
  'undefined'
]

var RESERVED_MAP = {}

for (var i = 0; i < RESERVED.length; i++) {
  RESERVED_MAP[RESERVED[i]] = true
}

var isVariable = function (name) {
  return isProperty(name) && !RESERVED_MAP.hasOwnProperty(name)
}

var formats = {
  s: function(s) {
    return '' + s
  },
  d: function(d) {
    return '' + Number(d)
  },
  o: function(o) {
    return JSON.stringify(o)
  }
}

var genfun = function() {
  var lines = []
  var indent = 0
  var vars = {}

  var push = function(str) {
    var spaces = ''
    while (spaces.length < indent*2) spaces += '  '
    lines.push(spaces+str)
  }

  var pushLine = function(line) {
    if (INDENT_END.test(line.trim()[0]) && INDENT_START.test(line[line.length-1])) {
      indent--
      push(line)
      indent++
      return
    }
    if (INDENT_START.test(line[line.length-1])) {
      push(line)
      indent++
      return
    }
    if (INDENT_END.test(line.trim()[0])) {
      indent--
      push(line)
      return
    }

    push(line)
  }

  var line = function(fmt) {
    if (!fmt) return line

    if (arguments.length === 1 && fmt.indexOf('\n') > -1) {
      var lines = fmt.trim().split('\n')
      for (var i = 0; i < lines.length; i++) {
        pushLine(lines[i].trim())
      }
    } else {
      pushLine(util.format.apply(util, arguments))
    }

    return line
  }

  line.scope = {}
  line.formats = formats

  line.sym = function(name) {
    if (!name || !isVariable(name)) name = 'tmp'
    if (!vars[name]) vars[name] = 0
    return name + (vars[name]++ || '')
  }

  line.property = function(obj, name) {
    if (arguments.length === 1) {
      name = obj
      obj = ''
    }

    name = name + ''

    if (isProperty(name)) return (obj ? obj + '.' + name : name)
    return obj ? obj + '[' + JSON.stringify(name) + ']' : JSON.stringify(name)
  }

  line.toString = function() {
    return lines.join('\n')
  }

  line.toFunction = function(scope) {
    if (!scope) scope = {}

    var src = 'return ('+line.toString()+')'

    Object.keys(line.scope).forEach(function (key) {
      if (!scope[key]) scope[key] = line.scope[key]
    })

    var keys = Object.keys(scope).map(function(key) {
      return key
    })

    var vals = keys.map(function(key) {
      return scope[key]
    })

    return Function.apply(null, keys.concat(src)).apply(null, vals)
  }

  if (arguments.length) line.apply(null, arguments)

  return line
}

genfun.formats = formats
module.exports = genfun