Make it easier to create a symbol from selected layers
Currently, it's very painful to create a symbol with the JS api. Its much easier using the internal method:
```
let sketch = require('sketch')
let doc = sketch.getSelectedDocument()
let selectedLayersObject = doc.selectedLayers
let selectedLayersArray = selectedLayersObject.layers

let interalLayersArray = selectedLayersArray.map(layer => layer.sketchObject)
// convert your JS layers to internal Sketch Objects

let msLayerArray = MSLayerArray.arrayWithLayers(interalLayersArray)
// MSSymbolCreator actually needs this MSLayerArray object, not just a NSArray of MSLayers for some reason
MSSymbolCreator.createSymbolFromLayers_withName_onSymbolsPage(msLayerArray,"test", true)
```

`MSSymbolCreator` handles a bunch of stuff like creating a symbols page if it doesn’t exist. It also sends the Symbol to the symbols page. It also creates a new instance of the symbol in place of the layer. It would be great if this method was exposed to the JS api.

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Related posts
- https://sketchplugins.com/d/1993-how-to-create-symbol
- https://sketchplugins.com/d/1945-get-selected-layers-after-selecting-programmatically/4