Add ndjson-cat.

README.md

@@ -8,6 +8,7 @@ npm install ndjson-cli
 
 ## Command Line Reference
 
+* [ndjson-cat](#ndjson_cat) - concatenate objects to form a stream
 * [ndjson-filter](#ndjson_filter) - filter objects
 * [ndjson-map](#ndjson_map) - transform objects
 * [ndjson-reduce](#ndjson_reduce) - reduce a stream of objects to a single value
@@ -16,6 +17,14 @@ npm install ndjson-cli
 * [ndjson-sort](#ndjson_sort) - sort a stream of objects
 * [ndjson-top](#ndjson_top) - select the top objects from a stream
 
+<a name="ndjson_cat" href="#ndjson_cat">#</a> <b>ndjson-cat</b> [<i>files…</i>] [<>](https://github.com/mbostock/ndjson-cli/blob/master/ndjson-cat "Source")
+
+Sequentially concatenates one or more input *files* containing JSON into a single newline-delimited JSON on stdout. If *files* is not specified, it defaults to “-”, indicating stdin. This command is especially useful for converting pretty-printed JSON (that contains newlines) into newline-delimited JSON. For example, to print the binaries exported by this repository’s package.json:
+
+```
+ndjson-cat package.json | ndjson-split 'Object.keys(d.bin)'
+```
+
 <a name="ndjson_filter" href="#ndjson_filter">#</a> <b>ndjson-filter</b> [<i>expression</i>] [<>](https://github.com/mbostock/ndjson-cli/blob/master/ndjson-filter "Source")
 
 Filters the newline-delimited JSON stream on stdin according to the specified *expression*: if the *expression* evaluates truthily for the given JSON object *d* at the given zero-based index *i* in the stream, the resulting JSON object is output to stdout; otherwise, it is ignored. If *expression* is not specified, it defaults to `true`. This program is much like [*array*.filter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter).

ndjson-cat

@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+
+var fs = require("fs"),
+    output = require("./output");
+
+if (process.argv.length < 3) {
+  process.argv[2] = "-";
+}
+
+var i = 1, n = process.argv.length, task = Promise.resolve();
+while (++i < n) task = task.then(cat(process.argv[i]));
+task.catch(function(error) { console.error(error); });
+
+function cat(input) {
+  return new Promise(function(resolve, reject) {
+    var data = [], stream = input === "-" ? process.stdin : fs.createReadStream(input);
+    stream
+        .on("data", function(d) { data.push(d); })
+        .on("error", reject)
+        .on("end", function() {
+          try {
+            var d = JSON.parse(data);
+          } catch (e) {
+            console.error((input === "-" ? "stdin" : input) + ": SyntaxError: " + e.message);
+            process.exit(1);
+          };
+          output(d);
+          resolve();
+        });
+  });
+}

package.json

@@ -18,6 +18,7 @@
     "url": "http://github.com/mbostock/ndjson-cli.git"
   },
   "bin": {
+    "ndjson-cat": "ndjson-cat",
     "ndjson-filter": "ndjson-filter",
     "ndjson-join": "ndjson-join",
     "ndjson-map": "ndjson-map",