add basic JS support by using custom Tensor replacement

The replacement is just a `seq[T]`

src/datamancer.nim

@@ -1,4 +1,8 @@
 ## .. include:: ./docs/datamancer.rst
 
-import datamancer / [dataframe, io]
-export dataframe, io
+when not defined(js):
+  import datamancer / [dataframe, io]
+  export dataframe, io
+else:
+  import datamancer / [dataframe, io]
+  export dataframe, io

src/datamancer/column.nim

@@ -1,4 +1,8 @@
-import arraymancer/tensor
+when not defined(js):
+  import arraymancer/tensor
+else:
+  import seq_tensor
+
 import std / [sugar, strformat, tables, macros, strutils]
 import value
 

src/datamancer/dataframe.nim

@@ -1,8 +1,12 @@
 import std / [macros, tables, strutils, options, sets, hashes, math,
               sequtils, stats, strformat, algorithm, typetraits]
 
-import arraymancer/tensor
-export tensor
+when not defined(js):
+  import arraymancer/tensor
+  export tensor
+else:
+  import seq_tensor
+  export seq_tensor
 
 import value
 export value
@@ -1989,7 +1993,7 @@ proc innerJoin*[C: ColumnLike](df1, df2: DataTable[C], by: string): DataTable[C]
       for k in getKeys(result):
         if result[k].kind != colConstant: # if constant nothing to short
           withNativeTensor(result[k], t):
-            result.asgn(k, toColumn(t[_ ..< result.len]))
+            result.asgn(k, toColumn(t[0 ..< result.len]))
         result[k].len = result.len
 
 proc innerJoin*[C: ColumnLike](dfs: varargs[DataTable[C]], by: string): DataTable[C] =

src/datamancer/formulaExp.nim

@@ -709,8 +709,11 @@ proc convertLoop(p: Preface, dtype, fctColResType, loop: NimNode,
                  fnKind: FormulaKind,
                  generateLoop: bool): NimNode =
   let memCopyable = ["float", "int", "bool"]
-  let isMemCopyable = dtype.strVal in memCopyable and
-    p.args.allIt(it.colType.strVal in memCopyable)
+  when defined(js):
+    let isMemcopyable = false
+  else:
+    let isMemCopyable = dtype.strVal in memCopyable and
+      p.args.allIt(it.colType.strVal in memCopyable)
   proc genForLoop(p: Preface, loop: NimNode, fkKind: FormulaKind): NimNode =
     var mpreface = p
     let loopIndexed = fixupTensorIndices(loop, mpreface,
@@ -765,6 +768,7 @@ proc convertLoop(p: Preface, dtype, fctColResType, loop: NimNode,
                                     rbKind = rbColumn)
     let resultId = ident(ResultIdent)
     let resId = ident(ResIdent)
+    echo p
     if generateLoop:
       ## raise error if no `Assign` known as  `ResIdent`
       if not p.hasResIdent: