Check that layers array items are objects (#1026)

* Fix unhandled error when layers contains primitive

Fixes an unhandled error occurring when a non-object type value is used
as a layer item, e.g. `layers: [123]`.

For primitive types, this caused an error such as:

> TypeError: Cannot use 'in' operator to search for 'ref' in 123

For `null` values, this caused an error such as:

> TypeError: Cannot read properties of null (reading 'type')

* Update CHANGELOG

Author: cxcorp

CHANGELOG.md

@@ -5,6 +5,7 @@
 
 ### 🐞 Bug fixes
 - Fix RuntimeError class, make it inherited from Error ([#983](https://github.com/maplibre/maplibre-style-spec/issues/983))
+- Validate that `layers` array items are objects instead of throwing an error if not ([!1026](https://github.com/maplibre/maplibre-style-spec/pull/1026))
 
 ## 23.1.0
 

src/validate/validate_layer.test.ts

@@ -0,0 +1,20 @@
+import {validate} from './validate';
+import {validateLayer} from './validate_layer';
+import v8 from '../reference/v8.json' with {type: 'json'};
+
+test.each([
+    ['number', 1],
+    ['string', '__proto__'],
+    ['boolean', true],
+    ['array', [{}]],
+    ['null', null],
+])('Should return error if layer value is %s instead of an object', (valueType, value) => {
+    const errors = validateLayer({
+        validateSpec: validate,
+        value: value,
+        styleSpec: v8,
+        style: {} as any,
+    });
+    expect(errors).toHaveLength(1);
+    expect(errors[0].message).toBe(`object expected, ${valueType} found`);
+});

src/validate/validate_layer.ts

@@ -6,6 +6,7 @@ import {validateFilter} from './validate_filter';
 import {validatePaintProperty} from './validate_paint_property';
 import {validateLayoutProperty} from './validate_layout_property';
 import {extendBy as extend} from '../util/extend';
+import {getType} from '../util/get_type';
 
 export function validateLayer(options) {
     let errors = [];
@@ -15,6 +16,10 @@ export function validateLayer(options) {
     const style = options.style;
     const styleSpec = options.styleSpec;
 
+    if (getType(layer) !== 'object') {
+        return [new ValidationError(key, layer, `object expected, ${getType(layer)} found`)]
+    }
+
     if (!layer.type && !layer.ref) {
         errors.push(new ValidationError(key, layer, 'either "type" or "ref" is required'));
     }