forEach, in sequence; map, in parallel

zoubin · Oct 13, 2015 · c99007f · c99007f
1 parent 79fef05
commit c99007f
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diff --git a/README.md b/README.md
@@ -14,6 +14,7 @@ Methods:
 
 - [filter](#filter)
 - [map](#map)
+- [forEach](#forEach)
 - [reduce](#reduce)
 - [chain](#chain)
 
@@ -35,32 +36,62 @@ filter(
   function (v) {
     return new Promise(function (rs) {
       process.nextTick(function () {
-        rs(v % 2);
-      });
-    });
+        rs(v % 2)
+      })
+    })
   },
   function (err, results) {
-    console.log('promise:', results);
+    console.log('promise:', results)
   }
-);
+)
 ```
 
 ## map
 
+`fn` is called with elements in parallel.
+If you want to run `fn` in sequence,
+use [forEach](#forEach) instead.
+
 Signature: `map(arr, fn, done)`
 
 ```javascript
 map(
   [1, 2, 3, 4],
   function (v, i, a, next) {
     process.nextTick(function () {
-      next(null, v << 2);
-    });
+      next(null, v << 2)
+    })
   },
   function (err, results) {
-    console.log('async:', results);
+    console.log('async:', results)
   }
-);
+)
+
+```
+
+## forEach
+
+`fn` is called with elements in sequence.
+If you want to run `fn` in parallel,
+use [map](#map) instead.
+
+Signature: `forEach(arr, fn, done)`
+
+```javascript
+var count = 0
+
+forEach(
+  [1, 2, 3, 4],
+  function (v, i, _, next) {
+    process.nextTick(function () {
+      console.log(count++ === i)
+      next(null, v << 2)
+    })
+  },
+  function (err, results) {
+    console.log(results)
+  }
+)
 
 ```
 
@@ -73,13 +104,13 @@ reduce(
   [1, 2, 3, 4],
   function (a, b, i, arr, next) {
     process.nextTick(function () {
-      next(null, a + b);
-    });
+      next(null, a + b)
+    })
   },
   function (err, results) {
-    console.log('async:', results);
+    console.log('async:', results)
   }
-);
+)
 
 ```
 
@@ -123,21 +154,21 @@ methods.chain(
 )
 
 function odd(v) {
-  return v % 2;
+  return v % 2
 }
 
 function plusplus(v, i, a, next) {
   process.nextTick(function () {
-    next(null, ++v);
-  });
+    next(null, ++v)
+  })
 }
 
 function sum(a, b) {
   return new Promise(function (rs) {
     process.nextTick(function () {
-      rs(a + b);
-    });
-  });
+      rs(a + b)
+    })
+  })
 }
 
 ```

diff --git a/example/forEach.js b/example/forEach.js
@@ -0,0 +1,17 @@
+var forEach = require('..').forEach
+
+var count = 0
+
+forEach(
+  [1, 2, 3, 4],
+  function (v, i, _, next) {
+    process.nextTick(function () {
+      console.log(count++ === i)
+      next(null, v << 2)
+    })
+  },
+  function (err, results) {
+    console.log(results)
+  }
+)
+
diff --git a/index.js b/index.js
@@ -1,5 +1,6 @@
 
-exports.forEach = exports.map = require('./lib/map')
+exports.forEach = require('./lib/forEach')
+exports.map = require('./lib/map')
 exports.filter = require('./lib/filter')
 exports.reduce = require('./lib/reduce')
 exports.chain = require('./lib/chain')

diff --git a/lib/each.js → lib/forEach.js b/lib/each.js → lib/forEach.js
@@ -1,6 +1,10 @@
 var run = require('run-callback')
 
-module.exports = function each(arr, fn, start, end, res, done) {
+module.exports = function (arr, fn, done) {
+  each(arr, fn, 0, arr.length, [], done)
+}
+
+function each(arr, fn, start, end, res, done) {
   if (end <= start) {
     return done(null, res)
   }

diff --git a/lib/map.js b/lib/map.js
@@ -1,6 +1,23 @@
-var each = require('./each')
+var run = require('run-callback')
 
 module.exports = function (arr, fn, done) {
-  each(arr, fn, 0, arr.length, [], done)
+  var pending = arr.length
+  var results = []
+  var errored = false
+  arr.forEach(function (v, i) {
+    run([fn, v, i, arr], function (err, r) {
+      if (errored) {
+        return
+      }
+      if (err) {
+        errored = true
+        return done(err)
+      }
+      results[i] = r
+      if (--pending === 0) {
+        done(null, results)
+      }
+    })
+  })
 }
 
diff --git a/lib/reduce.js b/lib/reduce.js
@@ -30,4 +30,3 @@ module.exports = function (arr, fn, initial, done) {
   }(start))
 }
 
-
diff --git a/test/forEach.js b/test/forEach.js
@@ -0,0 +1,42 @@
+var test = require('tape')
+var each = require('..').forEach
+
+test('forEach', function(t, cb) {
+  var count = 0
+  each(
+    [1, 2, 3, 4],
+    function (v, i, a, next) {
+      process.nextTick(function () {
+        t.equal(count++, i)
+        next(null, v << 2)
+      })
+    },
+    function (err, results) {
+      t.same(results, [4, 8, 12, 16])
+      cb()
+    }
+  )
+
+})
+
+test('forEach, err', function(t) {
+  t.plan(1)
+  each(
+    ['1', 2, '3', 4],
+    function (v, i, a, next) {
+      process.nextTick(function () {
+        try {
+          var code = v.charCodeAt(0)
+        } catch (e) {
+          return next(e)
+        }
+        next(null, code)
+      })
+    },
+    function (err) {
+      t.ok(err instanceof Error)
+    }
+  )
+
+})
+