Contents:

• JavaScript有效标识符命名规则：首字符为字母、_或$，后续可包含数字；区分大小写；保留字不可用。无效变量：以数字开头、含特殊字符（如-、空格）、使用保留关键字。
• 声明JavaScript变量的合理方法：默认使用const，需重新赋值时用let，避免var。优先块级作用域，保持作用域最小化，确保代码清晰可维护。
• Hoisting
• 变量求值
• JavaScript：数据类型
• JavaScript：表达式
• JavaScript：保留字

// Bad Practice
var foo = 1,
    bar = 2;

var baz;
var pony;

var x
  , y;
//------------------------------
var foo = 1;

if (foo > 1) {
  var bar = 2;
}

// Good  Practice
var foo = 1;
var bar = 2;

var baz;
var pony;

var x;
var y;

//------------------------------
var foo = 1;
var bar;

if (foo > 1) {
  bar = 2;
}

// bad
superPower = new SuperPower();

// good
var superPower = new SuperPower();

// bad
function() {
  test();
  console.log('doing stuff..');

  //..other stuff..

  var name = getName();

  if (name === 'test') {
    return false;
  }

  return name;
}

// good
function() {
  var name = getName();

  test();
  console.log('doing stuff..');

  //..other stuff..

  if (name === 'test') {
    return false;
  }

  return name;
}

// bad - unnecessary function call
function() {
  var name = getName();

  if (!arguments.length) {
    return false;
  }

  this.setFirstName(name);

  return true;
}

// good
function() {
  var name;

  if (!arguments.length) {
    return false;
  }

  name = getName();
  this.setFirstName(name);

  return true;
}

// we know this wouldn't work (assuming there
// is no notDefined global variable)
function example() {
  console.log(notDefined); // => throws a ReferenceError
}

// creating a variable declaration after you
// reference the variable will work due to
// variable hoisting. Note: the assignment
// value of `true` is not hoisted.
function example() {
  console.log(declaredButNotAssigned); // => undefined
  var declaredButNotAssigned = true;
}

// The interpreter is hoisting the variable
// declaration to the top of the scope,
// which means our example could be rewritten as:
function example() {
  var declaredButNotAssigned;
  console.log(declaredButNotAssigned); // => undefined
  declaredButNotAssigned = true;
}

function example() {
  console.log(anonymous); // => undefined

  anonymous(); // => TypeError anonymous is not a function

  var anonymous = function() {
    console.log('anonymous function expression');
  };
}

function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  superPower(); // => ReferenceError superPower is not defined

  var named = function superPower() {
    console.log('Flying');
  };
}

// the same is true when the function name
// is the same as the variable name.
function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  var named = function named() {
    console.log('named');
  }
}

var x;
  print("The value of x is : " + x); // prints "The value of x is : undefined" 
  print("The value of y is : " + y); // throws reference error exception 

var x;
if(x === undefined)
{ 
  do something ;
} 
else
{    
  do something ;
}

if (typeof abc === 'undefined') {
    // variable abc is undefined
}

const country = 'India';
//You cannot declare a constant with the same name as a function
// or variable in the same scope. Following statements create error.
function abc()
{
const abc = 55; 
}
function abc()
{ 
const x = 15; 
var x; 
}
 

var age = 21 

age = "What is your age?"

x = "What is your age ?" + 21 // returns "What is your age ?21  "
  y = 21 + " is my age. " // returns "21 is my age."

 "21" - 7 // returns 14 
  "21" + 7 // returns "217"
  "21" + "7" // returns "217"

"21" * 7 // returns 147
  "21" / 7 // returns 3