doc: update and modernize examples in api docs

doc/api/addons.markdown

@@ -99,7 +99,7 @@ You can now use the binary addon in a Node.js project `hello.js` by pointing
 `require` to the recently built `hello.node` module:
 
     // hello.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     console.log(addon.hello()); // 'world'
 
@@ -189,7 +189,7 @@ function calls and return a result. This is the main and only needed source
 You can test it with the following JavaScript snippet:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     console.log( 'This should be eight:', addon.add(3,5) );
 
@@ -237,7 +237,7 @@ adding the function as a property of `exports`.
 To test it, run the following JavaScript snippet:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     addon(function(msg){
       console.log(msg); // 'hello world'
@@ -282,7 +282,7 @@ the string passed to `createObject()`:
 To test it in JavaScript:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     var obj1 = addon('hello');
     var obj2 = addon('world');
@@ -336,7 +336,7 @@ wraps a C++ function:
 To test:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     var fn = addon();
     console.log(fn()); // 'hello world'
@@ -470,7 +470,7 @@ prototype:
 Test it with:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     var obj = new addon.MyObject(10);
     console.log( obj.plusOne() ); // 11
@@ -630,7 +630,7 @@ The implementation is similar to the above in `myobject.cc`:
 Test it with:
 
     // test.js
-    var createObject = require('./build/Release/addon');
+    const createObject = require('./build/Release/addon');
 
     var obj = createObject(10);
     console.log( obj.plusOne() ); // 11
@@ -792,7 +792,7 @@ The implementation of `myobject.cc` is similar to before:
 Test it with:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
     var obj1 = addon.createObject(10);
     var obj2 = addon.createObject(20);
@@ -866,7 +866,7 @@ The file `addon.cc` implements AtExit below:
 Test in JavaScript by running:
 
     // test.js
-    var addon = require('./build/Release/addon');
+    const addon = require('./build/Release/addon');
 
 [online]: https://v8docs.nodesource.com/
 [libuv]: https://github.com/libuv/libuv

doc/api/assert.markdown

@@ -41,7 +41,7 @@ assertion.
 
     assert.doesNotThrow(
       function() {
-        throw new TypeError("Wrong value");
+        throw new TypeError('Wrong value');
       },
       SyntaxError
     );
@@ -51,7 +51,7 @@ is thrown instead.
 
     assert.doesNotThrow(
       function() {
-        throw new TypeError("Wrong value");
+        throw new TypeError('Wrong value');
       },
       TypeError
     );
@@ -102,7 +102,7 @@ Validate instanceof using constructor:
 
     assert.throws(
       function() {
-        throw new Error("Wrong value");
+        throw new Error('Wrong value');
       },
       Error
     );
@@ -111,7 +111,7 @@ Validate error message using [`RegExp`][]:
 
     assert.throws(
       function() {
-        throw new Error("Wrong value");
+        throw new Error('Wrong value');
       },
       /value/
     );
@@ -120,19 +120,19 @@ Custom error validation:
 
     assert.throws(
       function() {
-        throw new Error("Wrong value");
+        throw new Error('Wrong value');
       },
       function(err) {
         if ( (err instanceof Error) && /value/.test(err) ) {
           return true;
         }
       },
-      "unexpected error"
+      'unexpected error'
     );
 
 [`assert.deepEqual`]: #assert_assert_deepequal_actual_expected_message
 [`assert.deepStrictEqual`]: #assert_assert_deepstrictequal_actual_expected_message
 [`assert.throws()`]: #assert_assert_throws_block_error_message
 [`Error`]: errors.html#errors_class_error
 [`RegExp`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
-[`TypeError`]: errors.html#errors_class_typeerror
+[`TypeError`]: errors.html#errors_class_typeerror

doc/api/buffer.markdown

@@ -104,8 +104,8 @@ Example:
 
     str = '\u00bd + \u00bc = \u00be';
 
-    console.log(str + ": " + str.length + " characters, " +
-      Buffer.byteLength(str, 'utf8') + " bytes");
+    console.log(`${str}: ${str.length} characters, ` +
+                `${Buffer.byteLength(str, 'utf8')} bytes`);
 
     // ½ + ¼ = ¾: 9 characters, 12 bytes
 
@@ -277,7 +277,7 @@ and `end` (defaults to `buffer.length`) are not given it will fill the entire
 buffer.
 
     var b = new Buffer(50);
-    b.fill("h");
+    b.fill('h');
 
 ### buf.indexOf(value[, byteOffset])
 
@@ -314,7 +314,7 @@ buffer object.  It does not change when the contents of the buffer are changed.
     buf = new Buffer(1234);
 
     console.log(buf.length);
-    buf.write("some string", 0, "ascii");
+    buf.write('some string', 0, 'ascii');
     console.log(buf.length);
 
     // 1234
@@ -326,7 +326,7 @@ modify the length of a buffer should therefore treat `length` as read-only and
 use [`buf.slice`][] to create a new buffer.
 
     buf = new Buffer(10);
-    buf.write("abcdefghj", 0, "ascii");
+    buf.write('abcdefghj', 0, 'ascii');
     console.log(buf.length); // 10
     buf = buf.slice(0,5);
     console.log(buf.length); // 5
@@ -652,7 +652,7 @@ The method will not write partial characters.
 
     buf = new Buffer(256);
     len = buf.write('\u00bd + \u00bc = \u00be', 0);
-    console.log(len + " bytes: " + buf.toString('utf8', 0, len));
+    console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
 
 ### buf.writeDoubleBE(value, offset[, noAssert])
 ### buf.writeDoubleLE(value, offset[, noAssert])