JS 语法快速参考

var a; // undefined
a + 2; // 值是 NaN

var n = null;
n * 32;			// 值是 0

const MY_OBJECT = {'key': 'value'};
MY_OBJECT.key = 'otherValue';

const MY_ARRAY = ['HTML','CSS'];
MY_ARRAY.push('JAVASCRIPT');
console.log(MY_ARRAY); //logs ['HTML','CSS','JAVASCRIPT'];

var answer = 42;
answer = 'Thanks for all the fish...';

'37' - 7 // 30
'37' + 7 // "377"

'1.1' + '1.1' // '1.11.1'
(+'1.1') + (+'1.1') // 2.2   
// Note: the parentheses are added for clarity, not required.

var car = { myCar: 'Saturn', getCar: carTypes('Honda'), special: sales };
console.log(car.myCar);   // Saturn
console.log(car.getCar);  // Honda
console.log(car.special); // Toyota

{
  // ...
}

var b = new Boolean(false);
if (b) // this condition evaluates to true
if (b == true) // this condition evaluates to false

throw 'Error2';   // String type
throw 42;         // Number type
throw true;       // Boolean type
throw {toString: function() { return "I'm an object!"; } };

function doSomethingErrorProne() {
  if (ourCodeMakesAMistake()) {
    throw (new Error('The message'));
  } else {
    doSomethingToGetAJavascriptError();
  }
}

try {
  doSomethingErrorProne();
} catch (e) {
  console.log(e.name); // logs 'Error'
  console.log(e.message); // logs 'The message' or a JavaScript error message)
}

function imgLoad(url) {
  return new Promise(function(resolve, reject) {
    var request = new XMLHttpRequest();
    request.open('GET', url);
    request.responseType = 'blob';
    request.onload = function() {
      if (request.status === 200) {
        resolve(request.response);
      } else {
        reject(Error('Image didn\'t load successfully; error code:'
                     + request.statusText));
      }
    };
    request.onerror = function() {
      reject(Error('There was a network error.'));
    };
    request.send();
  });
}

var square = function(number) { return number * number; };
var x = square(4); // x gets the value 16

// 带函数名的函数表达式
var factorial = function fac(n) { return n < 2 ? 1 : n * fac(n - 1); };
console.log(factorial(3));

arguments[i]

function myConcat(separator) {
   var result = ''; // initialize list
   var i;
   // iterate through arguments
   for (i = 1; i < arguments.length; i++) {
      result += arguments[i] + separator;
   }
   return result;
}

// returns "red, orange, blue, "
myConcat(', ', 'red', 'orange', 'blue');
// returns "elephant; giraffe; lion; cheetah; "
myConcat('; ', 'elephant', 'giraffe', 'lion', 'cheetah');
// returns "sage. basil. oregano. pepper. parsley. "
myConcat('. ', 'sage', 'basil', 'oregano', 'pepper', 'parsley');

function multiply(a, b = 1) {
  return a * b;
}

multiply(5); // 5

function multiply(multiplier, ...theArgs) {
  return theArgs.map(x => multiplier * x);
}

var arr = multiply(2, 1, 2, 3);
console.log(arr); // [2, 4, 6]

'use strict';
var v = "Hi! I'm a strict mode script!";

function Person() {
  var self = this; // Some choose `that` instead of `self`. 
                   // Choose one and be consistent.
  self.age = 0;

  setInterval(function growUp() {
    // The callback refers to the `self` variable of which
    // the value is the expected object.
    self.age++;
  }, 1000);
}

function Person() {
  this.age = 0;

  setInterval(() => {// doesn't have a 'this' pointer inside inner function.
    this.age++; // |this| properly refers to the person object
  }, 1000);
}

var p = new Person();

var foo = ['one', 'two', 'three'];

// without destructuring
var one   = foo[0];
var two   = foo[1];
var three = foo[2];

// with destructuring
var [one, two, three] = foo;

delete objectName;
delete objectName.property;
delete objectName[index];
delete property; // legal only within a with statement

var myFun = new Function('5 + 2');
var shape = 'round';
var size = 1;
var foo = ['Apple', 'Mango', 'Orange'];
var today = new Date();
typeof myFun;       // returns "function"
typeof shape;       // returns "string"
typeof size;        // returns "number"
typeof foo;         // returns "object"
typeof today;       // returns "object"
typeof doesntExist; // returns "undefined"

typeof true; // returns "boolean"
typeof null; // returns "object"

// where propNameOrNumber is a string, numeric, or symbol expression representing a property name or array index, and objectName is the name of an object.
propNameOrNumber in objectName

// 另外还有一些有趣的用法:

// Arrays
var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
0 in trees;        // returns true
3 in trees;        // returns true
6 in trees;        // returns false
'bay' in trees;    // returns false (you must specify the index number,
                   // not the value at that index)
'length' in trees; // returns true (length is an Array property)

// built-in objects
'PI' in Math;          // returns true
var myString = new String('coral');
'length' in myString;  // returns true

// Custom objects
var mycar = { make: 'Honda', model: 'Accord', year: 1998 };
'make' in mycar;  // returns true
'model' in mycar; // returns true

var theDay = new Date(1995, 12, 17);
if (theDay instanceof Date) {
  // statements to execute
}

var biggestNum = Number.MAX_VALUE;
var smallestNum = Number.MIN_VALUE;
var infiniteNum = Number.POSITIVE_INFINITY;
var negInfiniteNum = Number.NEGATIVE_INFINITY;
var notANum = Number.NaN;

/*
JavaScript is designed on a simple object-based paradigm. An object is a collection of properties, and a property is an association between a name (or key) and a value. A property's value can be a function, in which case the property is known as a method. In addition to objects that are predefined in the browser, you can define your own objects.

JS 中的对象就是属性的集合. 属性实际就是一个名字和值的关联, 属性的值可以是函数(这个属性就变成了对象方法).
*/

function instanceOf(object, constructor) {
 object = object.__proto__;
 while (object != null) {
    if (object == constructor.prototype)
       return true;
    if (typeof object == 'xml') {
      return constructor.prototype == XML.prototype;
    }
    object = object.__proto__;
 }
 return false;
}