lib: fix readFile flag option

lib/fs.js

@@ -322,7 +322,7 @@ function readFile(path, options, callback) {
     return;
   }
 
-  const flagsNumber = stringToFlags(options.flags);
+  const flagsNumber = stringToFlags(options.flag);
   path = getValidatedPath(path);
 
   const req = new FSReqCallback();

test/parallel/test-fs-readfile-flags.js

@@ -0,0 +1,51 @@
+'use strict';
+
+// Test of fs.readFile with different flags.
+const common = require('../common');
+const fs = require('fs');
+const assert = require('assert');
+const path = require('path');
+const tmpdir = require('../common/tmpdir');
+
+tmpdir.refresh();
+
+{
+  const emptyFile = path.join(tmpdir.path, 'empty.txt');
+  fs.closeSync(fs.openSync(emptyFile, 'w'));
+
+  fs.readFile(
+    emptyFile,
+    // With `a+` the file is created if it does not exist
+    { encoding: 'utf8', flag: 'a+' },
+    common.mustCall((err, data) => { assert.strictEqual(data, ''); })
+  );
+
+  fs.readFile(
+    emptyFile,
+    // Like `a+` but fails if the path exists.
+    { encoding: 'utf8', flag: 'ax+' },
+    common.mustCall((err, data) => { assert.strictEqual(err.code, 'EEXIST'); })
+  );
+}
+
+{
+  const willBeCreated = path.join(tmpdir.path, 'will-be-created');
+
+  fs.readFile(
+    willBeCreated,
+    // With `a+` the file is created if it does not exist
+    { encoding: 'utf8', flag: 'a+' },
+    common.mustCall((err, data) => { assert.strictEqual(data, ''); })
+  );
+}
+
+{
+  const willNotBeCreated = path.join(tmpdir.path, 'will-not-be-created');
+
+  fs.readFile(
+    willNotBeCreated,
+    // Default flag is `r`. An exception occurs if the file does not exist.
+    { encoding: 'utf8' },
+    common.mustCall((err, data) => { assert.strictEqual(err.code, 'ENOENT'); })
+  );
+}