Create a helper that normalizes a value to an array (if not already an array)

[oleq]

Provide a description of the task

We use the following normalization Array.isArray( foo ) ? foo : [ foo ]; many times across the code-base. I think we need an utils for this repeating pattern.

packages/ckeditor5-engine/src/conversion/conversion.js:
   77: 		this._downcast = Array.isArray( downcastDispatchers ) ? downcastDispatchers : [ downcastDispatchers ];
   80: 		this._upcast = Array.isArray( upcastDispatchers ) ? upcastDispatchers : [ upcastDispatchers ];
  640: 		upcastAlso = Array.isArray( upcastAlso ) ? upcastAlso : [ upcastAlso ];

packages/ckeditor5-engine/src/conversion/downcasthelpers.js:
  850: 				const classes = Array.isArray( oldAttribute.value ) ? oldAttribute.value : [ oldAttribute.value ];
  869: 				const classes = Array.isArray( newAttribute.value ) ? newAttribute.value : [ newAttribute.value ];

packages/ckeditor5-engine/src/view/element.js:
  730: 		className = Array.isArray( className ) ? className : [ className ];
  748: 		className = Array.isArray( className ) ? className : [ className ];
  795: 		property = Array.isArray( property ) ? property : [ property ];

packages/ckeditor5-image/src/image/imageinsertcommand.js:
  54: 			const sources = Array.isArray( options.source ) ? options.source : [ options.source ];

packages/ckeditor5-image/src/imageupload/imageuploadcommand.js:
  64: 			const filesToUpload = Array.isArray( options.file ) ? options.file : [ options.file ];

packages/ckeditor5-watchdog/src/contextwatchdog.js:
  237: 		const itemConfigurations = Array.isArray( itemConfigurationOrItemConfigurations ) ?
  313: 		const itemIds = Array.isArray( itemIdOrItemIds ) ?

packages/ckeditor5-widget/src/utils.js:
  120: 		return Array.isArray( classes ) ? classes : [ classes ];

[niegowski]

I was wondering why we are not using [].concat( classes ) ? Result is the same as for Array.isArray( classes ) ? classes : [ classes ]

[oleq]

Not a very versatile test but this is probably the reason why

[niegowski]

Not a very versatile test but this is probably the reason why

And for classes list... do we ever expect it to be more than 2 or 3? 😉

Ok, it's not the case of how many classes but how many calls

[oleq]

AFAIR @jodator researched this in the past so maybe he would like to chime in.

[jodator]

From the performance perspective it might be a case that Array.concat() might do many things under the hood (checking for params types, if it is array then do other things, etc) and the simple call is faster (but longer in code) for this case.