Pass computed default value to slate.config.js functions

packages/slate-config/__tests__/index.test.js

@@ -95,14 +95,35 @@ describe('SlateConfig.get()', () => {
     expect(config.get('some.other.key')).toBe(schema['some.other.key']);
   });
 
-  test('if the value is a function, the function is executed with the config instance as the only argument', () => {
+  test('if the value is a function and not specified in this.userConfig, the function is executed with the config instance as the only argument', () => {
     const SlateConfig = require('../index');
     const config = new SlateConfig(schema);
     const value = config.get('some.function');
 
     expect(schema['some.function']).toBeCalledWith(config);
     expect(value).toBe(schema['some.other.key']);
   });
+
+  test('if the value is specified in this.userConfig and is a function, it is executed with the config instance and the computed default value as arguments', () => {
+    const userConfigValue = 'someNewValue';
+    const userConfigFunction = jest.fn(() => userConfigValue);
+
+    const SlateConfig = require('../index');
+    const config = new SlateConfig(schema);
+    const defaultValue = config.get('some.function');
+
+    global.slateUserConfig = {
+      'some.function': userConfigFunction,
+    };
+
+    const value = config.get('some.function');
+
+    expect(global.slateUserConfig['some.function']).toBeCalledWith(
+      config,
+      defaultValue,
+    );
+    expect(value).toBe(userConfigValue);
+  });
 });
 
 describe('SlateConfig.set()', () => {

packages/slate-config/index.js

@@ -31,20 +31,31 @@ module.exports = class SlateConfig {
   }
 
   get(key) {
-    const userConfig = this.userConfig;
-    const value =
-      typeof userConfig[key] === 'undefined'
-        ? this.schema[key]
-        : userConfig[key];
-
-    if (typeof value === 'function') {
-      return value(this);
-    } else if (typeof value === 'undefined') {
+    const defaultValue = this.schema[key];
+    const userConfigValue = this.userConfig[key];
+    let computedDefaultValue;
+
+    if (
+      typeof defaultValue === 'undefined' &&
+      typeof userConfigValue === 'undefined'
+    ) {
       throw new Error(
         `[slate-config]: A value has not been defined for the key '${key}'`,
       );
+    }
+
+    if (typeof defaultValue === 'function') {
+      computedDefaultValue = defaultValue(this);
+    } else {
+      computedDefaultValue = defaultValue;
+    }
+
+    if (typeof userConfigValue === 'undefined') {
+      return computedDefaultValue;
+    } else if (typeof userConfigValue === 'function') {
+      return userConfigValue(this, computedDefaultValue);
     } else {
-      return value;
+      return userConfigValue;
     }
   }
 };