属性

使用点表示法访问属性

var luke = {
jedi: true,
age: 28
};

// bad
var isJedi = luke['jedi'];

// good
var isJedi = luke.jedi;

用变量访问属性时要使用下标表示法([])

var luke = {
  jedi: true,
  age: 28
};

function getProp(prop) {
  return luke[prop];
}

var isJedi = getProp('jedi');

变量

总是使用var声明变量，不然其将变为全局变量。我们要想办法避免全局空间污染

// bad
superPower = new SuperPower();

// good
var superPower = new SuperPower();

使用var声明每个变量，这样很容易添加新的变量声明，而不用去担心用a;替换a

// bad
var items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// bad
// (compare to above, and try to spot the mistake)
var items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// good
var items = getItems();
var goSportsTeam = true;
var dragonball = 'z';

最后声明未赋值的变量，这对于你需要根据之前已经赋值的变量对一个变量进行赋值时是很有帮助的

// bad
var i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
var i;
var items = getItems();
var dragonball;
var goSportsTeam = true;
var len;

// good
var items = getItems();
var goSportsTeam = true;
var dragonball;
var length;
var i;

在作用域顶端对变量赋值，这有助于避免变量声明问题和与声明提升相关的问题

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

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

  return true;
}

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

  var name = getName();

  return true;
}

声明提升

变量声明是在作用域的顶端，但是赋值没有

// 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;
}

匿名表达式能提升他们的变量名，但不能提升函数赋值

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

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

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

命名函数表达式会提升变量名，而不是函数名或者函数体

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');
  }
}

函数声明会提升变量名和函数体

function example() {
  superPower(); // => Flying

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

比较运算符&相等

使用===和!==代替==和!= 比较运算符进行计算时会利用ToBoolean方法进行强制转换数据类型，并遵从一下规则

1.Objects的计算值是true
2.Undefined的计算值是false
3.Boolean的计算值是boolean的值
4.Numbers如果是-0，+0或者NaN，则计算值是false,反之是true
5.Strings如果是空，则计算值是false,反之是true

if ([0]) {
  // true
  // An array is an object, objects evaluate to true
}

使用快捷方式

// bad
if (name !== '') {
  // ...stuff...
}

// good
if (name) {
  // ...stuff...
}

// bad
if (collection.length > 0) {
  // ...stuff...
}

// good
if (collection.length) {
  // ...stuff...
}

语句块

对多行的语句块使用大括号

// bad
if (test)
  return false;

// good
if (test) return false;

// good
if (test) {
  return false;
}

// bad
function() { return false; }

// good
function() {
  return false;
}

对于使用if和else的多行语句块，把else和if语句块的右大括号放在同一行

// bad
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}

// good
if (test) {
  thing1();
  thing2();
} else {
  thing3();
}