Allow conditional configuration via when

README.md

@@ -204,6 +204,16 @@ entries()
 merge(obj)
 ```
 
+```js
+// Conditionally execute a function to continue configuration
+// condition: Boolean
+// truthyHandler: Function -> ChainedMap
+  // invoked when condition is truthy, given a single argument of the ChainedMap instance
+// falsyHandler: Function -> ChainedMap
+  // invoked when condition is falsy, given a single argument of the ChainedMap instance
+when(condition, truthyHandler, falsyHandler)
+```
+
 ## ChainedSet
 
 Another of the core API interfaces in webpack-chain is a `ChainedSet`. A `ChainedSet` operates
@@ -255,6 +265,16 @@ values()
 merge(arr)
 ```
 
+```js
+// Conditionally execute a function to continue configuration
+// condition: Boolean
+// truthyHandler: Function -> ChainedSet
+  // invoked when condition is truthy, given a single argument of the ChainedSet instance
+// falsyHandler: Function -> ChainedSet
+  // invoked when condition is falsy, given a single argument of the ChainedSet instance
+when(condition, truthyHandler, falsyHandler)
+```
+
 ## Shorthand methods
 
 A number of shorthand methods exist for setting a value on a `ChainedMap`
@@ -845,3 +865,30 @@ config.merge({
   }
 })
 ```
+
+### Conditional configuration
+
+When working with instances of `ChainedMap` and `ChainedSet`, you can perform conditional configuration using `when`.
+You must specify an expression to `when()` which will be evaluated for truthiness or falsiness. If the expression is
+truthy, the first function argument will be invoked with an instance of the current chained instance. You can optionally
+provide a second function to be invoked when the condition is falsy, which is also given the current chained instance.
+
+```js
+// Example: Only add minify plugin during production
+config
+  .when(process.env.NODE_ENV === 'production', config => {
+    config
+      .plugin('minify')
+      .use(BabiliWebpackPlugin);
+  });
+```
+
+```js
+// Example: Only add minify plugin during production,
+// otherwise set devtool to source-map
+config
+  .when(process.env.NODE_ENV === 'production',
+    config => config.plugin('minify').use(BabiliWebpackPlugin),
+    config => config.devtool('source-map')
+  );
+```

src/ChainedMap.js

@@ -81,4 +81,14 @@ module.exports = class extends Chainable {
         return acc;
       }, {});
   }
+
+  when(condition, trueBrancher = Function.prototype, falseBrancher = Function.prototype) {
+    if (condition) {
+      trueBrancher(this);
+    } else {
+      falseBrancher(this);
+    }
+
+    return this;
+  }
 };

src/ChainedSet.js

@@ -38,4 +38,14 @@ module.exports = class extends Chainable {
     this.store = new Set([...this.store, ...arr]);
     return this;
   }
+
+  when(condition, trueBrancher = Function.prototype, falseBrancher = Function.prototype) {
+    if (condition) {
+      trueBrancher(this);
+    } else {
+      falseBrancher(this);
+    }
+
+    return this;
+  }
 };

test/ChainedMap.js

@@ -117,3 +117,33 @@ test('merge with overriding values', t => {
   t.is(map.merge(obj), map);
   t.deepEqual(map.entries(), { a: 'alpha', b: 'beta', c: 'gamma' });
 });
+
+test('when true', t => {
+  const map = new ChainedMap();
+  const right = instance => {
+    t.is(instance, map);
+    instance.set('alpha', 'a');
+  };
+  const left = instance => {
+    instance.set('beta', 'b');
+  };
+
+  t.is(map.when(true, right, left), map);
+  t.true(map.has('alpha'));
+  t.false(map.has('beta'));
+});
+
+test('when false', t => {
+  const map = new ChainedMap();
+  const right = instance => {
+    instance.set('alpha', 'a');
+  };
+  const left = instance => {
+    t.is(instance, map);
+    instance.set('beta', 'b');
+  };
+
+  t.is(map.when(false, right, left), map);
+  t.false(map.has('alpha'));
+  t.true(map.has('beta'));
+});

test/ChainedSet.js

@@ -95,3 +95,33 @@ test('merge with existing values', t => {
   t.is(set.merge(arr), set);
   t.deepEqual(set.values(), ['delta', 'alpha', 'beta', 'gamma']);
 });
+
+test('when true', t => {
+  const set = new ChainedSet();
+  const right = instance => {
+    t.is(instance, set);
+    instance.add('alpha');
+  };
+  const left = instance => {
+    instance.add('beta');
+  };
+
+  t.is(set.when(true, right, left), set);
+  t.true(set.has('alpha'));
+  t.false(set.has('beta'));
+});
+
+test('when false', t => {
+  const set = new ChainedSet();
+  const right = instance => {
+    instance.add('alpha');
+  };
+  const left = instance => {
+    t.is(instance, set);
+    instance.add('beta');
+  };
+
+  t.is(set.when(false, right, left), set);
+  t.false(set.has('alpha'));
+  t.true(set.has('beta'));
+});