js2coffee/lib/transforms/loops.coffee

JavaScript to CoffeeScript compiler

{ replace, isLoop } = require('../helpers')
TransformerBase = require('./base')

###
# Provides transformations for `while`, `for` and `do`.
###

module.exports = class extends TransformerBase

  ForStatement: (node) ->
    # Keep track of the nesting of `for` loops
    @_for ?= []
    @_for.push node

    @injectUpdateIntoBody(node)
    @convertForToWhile(node)

  ForStatementExit: (node) ->
    _for = @_for.pop()
    delete _for._update
    node

  ForInStatement: (node) ->
    @warnIfNoVar(node)

  WhileStatement: (node) ->
    @convertWhileToLoop(node)

  DoWhileStatement: (node) ->
    @convertDoWhileToLoop(node)

  ContinueStatement: (node, parent) ->
    @injectUpdateIntoContinue(node, parent)

  ###
  # In a `for` loop with an update statement, inject the update just
  # before the `continue`.
  #
  # This replaces the ContinueStatement with a block that groups the
  # update and continue together.
  ###

  injectUpdateIntoContinue: (node, parent) ->
    _for = @_for?[@_for.length-1]
    return node unless _for?._update

    @skip()

    type: 'BlockStatement'
    body: [
      {
        type: 'ExpressionStatement'
        expression: _for._update
      }
      node
    ]

  ###
  # Converts `do { x } while (y)` to `loop\  x\  break unless y`.
  ###

  convertDoWhileToLoop: (node) ->
    block = node.body
    body = block.body

    body.push replace node.test,
      type: 'IfStatement'
      _negative: true
      test: node.test
      consequent:
        type: 'BreakStatement'

    replace node,
      type: 'CoffeeLoopStatement'
      body: block

  ###
  # Produce a warning for `for (x in y)` where `x` is not `var x`.
  ###

  warnIfNoVar: (node) ->
    if node.left.type isnt 'VariableDeclaration'
      @warn node, "Using 'for..in' loops without " +
        "'var' can produce unexpected results"
    node

  ###
  # Converts a `for (x;y;z) {a}` to `x; while(y) {a; z}`.
  # Returns a `BlockStatement`.
  ###

  convertForToWhile: (node) ->
    node.type = if isLoop(node)
      'CoffeeLoopStatement'
    else
      'WhileStatement'

    if node.init?
      type: 'BlockStatement'
      body: [
        {
          type: 'ExpressionStatement'
          expression: node.init
        }
        node
      ]
    else
      node

  ###
  # Converts a `while (true)` to a CoffeeLoopStatement.
  ###

  convertWhileToLoop: (node) ->
    if isLoop(node)
      replace node,
        type: 'CoffeeLoopStatement'
        body: node.body
    else
      node

  ###*
  # Injects a ForStatement's update (eg, `i++`) into the body.
  ###

  injectUpdateIntoBody: (node) ->
    if node.update
      statement =
        type: 'ExpressionStatement'
        expression: node.update

      # Ensure that the body is a BlockStatement with a body
      if not node.body?
        node.body ?= { type: 'BlockStatement', body: [] }
      else if node.body.type isnt 'BlockStatement'
        old = node.body
        node.body = { type: 'BlockStatement', body: [ old ] }

      node.body.body = node.body.body.concat([statement])

      # leave it for `continue` to pick up
      node._update = node.update
      delete node.update
    node