Use CanvasLineJoin type instead of string

src/ol/style/RegularShape.js

@@ -33,7 +33,7 @@ import ImageStyle from './Image.js';
  * @property {string} lineCap
  * @property {Array<number>} lineDash
  * @property {number} lineDashOffset
- * @property {string} lineJoin
+ * @property {CanvasLineJoin} lineJoin
  * @property {number} miterLimit
  */
 
@@ -303,7 +303,7 @@ class RegularShape extends ImageStyle {
    */
   render() {
     let lineCap = '';
-    let lineJoin = '';
+    let lineJoin = defaultLineJoin;
     let miterLimit = 0;
     let lineDash = null;
     let lineDashOffset = 0;
@@ -418,7 +418,7 @@ class RegularShape extends ImageStyle {
         context.lineDashOffset = renderOptions.lineDashOffset;
       }
       context.lineCap = /** @type {CanvasLineCap} */ (renderOptions.lineCap);
-      context.lineJoin = /** @type {CanvasLineJoin} */ (renderOptions.lineJoin);
+      context.lineJoin = renderOptions.lineJoin;
       context.miterLimit = renderOptions.miterLimit;
       context.stroke();
     }

src/ol/style/Stroke.js

@@ -9,7 +9,7 @@
  * See {@link module:ol/color~Color} and {@link module:ol/colorlike~ColorLike} for possible formats.
  * Default null; if null, the Canvas/renderer default black will be used.
  * @property {string} [lineCap='round'] Line cap style: `butt`, `round`, or `square`.
- * @property {string} [lineJoin='round'] Line join style: `bevel`, `round`, or `miter`.
+ * @property {CanvasLineJoin} [lineJoin='round'] Line join style: `bevel`, `round`, or `miter`.
  * @property {Array<number>} [lineDash] Line dash pattern. Default is `undefined` (no dash).
  * Please note that Internet Explorer 10 and lower do not support the `setLineDash` method on
  * the `CanvasRenderingContext2D` and therefore this option will have no visual effect in these browsers.
@@ -61,7 +61,7 @@ class Stroke {
 
     /**
      * @private
-     * @type {string|undefined}
+     * @type {CanvasLineJoin|undefined}
      */
     this.lineJoin_ = options.lineJoin;
 
@@ -134,7 +134,7 @@ class Stroke {
 
   /**
    * Get the line join type for the stroke.
-   * @return {string|undefined} Line join.
+   * @return {CanvasLineJoin|undefined} Line join.
    * @api
    */
   getLineJoin() {
@@ -208,7 +208,7 @@ class Stroke {
   /**
    * Set the line join.
    *
-   * @param {string|undefined} lineJoin Line join.
+   * @param {CanvasLineJoin|undefined} lineJoin Line join.
    * @api
    */
   setLineJoin(lineJoin) {