ES6 Precautions

1. let && const

   • We have block-level scope
   • Unrepeatable statement
   • Temporary dead zone (scope statement before, unavailable, error)
   • The former is not declared, it is direct typeof error, but typeof as a defined variable does not complain output undefined (typeof no longer safe)
   • const, the statement must be assigned (not the value of the variable may not change, but the memory address of the variable points of the stored data may not change)
   • let, const, class declare global variables, property does not belong to the top-level object. ES6 from the beginning, will gradually decouple from the global variable properties of the top-level object.
   • globalThis global object (the proposal)

2. Deconstruction

   • The string can be deconstructed assignment. const [a, b, c, d, e] = 'hello';
   • When destructuring assignment, if the right side of the equal sign is a numerical and Boolean values, it will first turn objects

   	let {toString: s} = 123;
   	s === Number.prototype.toString
   

3. String Methods

   • includes(), startsWith(), endsWith() ,padStart()，padEnd(), padStart()，padEnd()

4. digital

   • Number.isFinite (), Number.isNaN () and isNaN () distinction, isNaN () will be converted to digital, such as isNaN ( 'a') // true
   • Number.isInteger(), Number.isSafeInteger()
   • The method used to determine the number of Math.sign in the end is a positive, negative, or zero
   • The exponential operator (**)

5. function

   • length property, meaning that the expected number of parameters passed to the function, and specifies the default value, this parameter is not statistics
   • rest parameters (in the form of a variable name ...), you can not have other parameters after the rest parameters
   • attribute name, function name var f = function () {}; // es5 f.name => "" // es6 f.name => "f"
   • this object within the arrow function is defined when the object is located
   • Arrow can not function as a constructor
   • Arrow can not function arguments object
   • Generator function can not be used as a function of the arrow

6. Array

   • Extended operator, copy the array, merge array
   • And destructuring assignment binding const [first, ... rest] = [1, 2, 3, 4, 5];
   • The string to a real array [... 'hello']
   • Iterator
   • Array.from the two types of objects into a true array: array-like objects and can traverse objects, Array.from (Set)
   • Array.of method for a set of values, into an array Array.of (3, 11, 8)
   • find () and findIndex ()
   • fill() // let a = []; a.length =100; a.fill(1) // [1,1,1,1,…100个1]
   • includes()
   • flat () / flatMap () flattened array
   • Vacancy array, ES6 is clear the space into undefined // Array (3) // [,,,]

7. Objects

   • Object.getOwnPropertyNames contains all the properties of the object itself (excluding Symbol properties, including but not enumerated attribute)
   • Object.getOwnPropertySymbols returns an array containing all the properties of the object itself Symbol keys
   • Reflect.ownKeys returns an array containing all the key names of the object itself, regardless of the key name is Symbol or string, whether or not enumerable
   • Extended operator // {... {b: 2}, a: 1} // {b: 2, a: 1}
   • Object.is like ===
   • Object.assign (target, source1, source2) the source object (source) of all enumerated attribute, copied to the target object (target) ( shallow copy )
   • Object.getPrototypeOf() => proto
   • Object.keys (), Object.values ​​(), Object.entries () (an array of key-value pairs)

8. set / map

   • Inside Set, two are equal NaN
   • Map traversal sequence is inserted in order

9. promise

   //	加载图片
   const preloadImage = function (path) {
     return new Promise(function (resolve, reject) {
       const image = new Image();
       image.onload  = resolve;
       image.onerror = reject;
       image.src = path;
     });
   };
   

   • Promise.all / Promise.race

10. Iterator

    • Any data structure as long as the deployment Iterator interface, to complete traversal for ... of
    • Object (Object) is no reason why the default deployment Iterator interface, which is because the properties of the object first traversal, after which traverse the property is uncertain
    • Get Iterator

    let iter = [1,2,3][Symbol.iterator]()
    iter.next()  //{value: 1, done: false}
    iter.next()  //{value: 2, done: false}
    iter.next()  //{value: 3, done: true}
    

• Iterator interface = call occasions> destructuring assignment, expansion operators, yield *, for ... of, Array.from (), Map (), Set (), Promise.all / Promise.race

11. async
    • async function can be seen as a plurality of asynchronous operations, packed into a Promise object, and then await the command syntax of the command is the internal sugar
    • Internal async function return value returned by the statement, will be the method parameters then the callback function
    • No return, returned resolve (undefined)
    • Internal async function throws an error will result Promise returned object becomes reject state.

    			async function f() {
    			  throw new Error('出错了');
    			}
    			
    			f().then(
    			  v => console.log(v),
    			).catch(e=>{
    				console.log(e)
    			})
    

    5. await command can only appear within the async function, otherwise it will error