src/vs/editor/common/config/editorOptions.ts/EditorRenderLineNumbersOption/validate
class EditorRenderLineNumbersOption:	public validate(lineNumbers: any): InternalEditorRenderLineNumbersOptions {
		let renderType: RenderLineNumbersType = this.defaultValue.renderType;
		let renderFn: ((lineNumber: number) => string) | null = this.defaultValue.renderFn;

		if (typeof lineNumbers !== 'undefined') {
			if (typeof lineNumbers === 'function') {
				renderType = RenderLineNumbersType.Custom;
				renderFn = lineNumbers;
			} else if (lineNumbers === 'interval') {
				renderType = RenderLineNumbersType.Interval;
			} else if (lineNumbers === 'relative') {
				renderType = RenderLineNumbersType.Relative;
			} else if (lineNumbers === 'on') {
				renderType = RenderLineNumbersType.On;
			} else {
				renderType = RenderLineNumbersType.Off;
			}
		}

		return {
			renderType,
			renderFn
		};
	}