demo on github

let&const

• let does not support variable promotion

   console.log(a); // Report here
   let a = "a";
  

• let cannot repeat declaration

   let a = "a";
   let a = "abc"; // Report errors
  

• let supports block level scope

   if (true) {
     let a = "a";
   }
   console.log(a) // Report errors
  
   for (let i = 0; i < 5; i++) {
     setTimeout(() => {
       console.log(i); // 0 1 2 3 4
     });
   }
  

• In addition to the let function, const cannot change the declared value, but it can add properties and change property values to the declared objects

   const PI = 3.14;
   PI = 3.141; // Report errors
  
   const obj = {
     name: 'Xiao Zhang'
   };
   obj.name = 'Xiaohong';
   obj.age = 25;
  

deconstruction

{
  // Array deconstruction assignment
  let [a, b, c] = [123, "abc", { name: "xiaohong" }];
  console.log(a, b, c); // 123 'abc' { name: 'xiaohong' }
}

{
  // Object deconstruction assignment
  let { name, age } = {
    name: "xiaohong",
    age: 25
  };

  console.log(name, age); // xiaohong 25
}

{
  // Custom name of value assigned to deconstruction
  let { name: myname, age: myage } = {
    name: "xiaohong",
    age: 25
  };

  console.log(myname, myage); // xiaohong 25
}

{
  // Default assignment. If assigned to age, the default value will be overwritten
  let { name, age = 19 } = {
    name: "xiaohong"
  };

  console.log(name, age); // xiaohong 19
}

{
  // Ellipsis assignment
  let [, , a] = [1, 2, 3];
  console.log(a); // 3
}

Expand operator

• Use expansion operators in functions

   function test(a, b, c) {}
   let arr = [1, 2, 3];
   test(...arr);

• Using expansion operators in functions in arrays

   let [a, b, ...c] = [1, 2, 3, 4, 5];
   console.log(a, b, c); // 1 2 [ 3, 4, 5 ]
  
   let arr1 = [1, 2, 3];
   let arr2 = [...arr1, 4, 5];
   console.log(arr2); // [ 1, 2, 3, 4, 5 ]

• Class array variable to array

  function test(a, b, c) {
     console.log([...arguments]);
   }
   test(1, 2, 4); // [1 2 4]
  

Character string

• Template string: before that, string splicing was completed with + sign. Now '` and S {} can replace string splicing

   let name = 'xiaohong', age = 25;
   let str = `My name is:${name},This year ${age}Year old`;
   console.log(str); // My name is xiaohong, 25 years old   
  
  {
   // Return value of custom template string
   let name = "xiaohong",
     age = 25;
   // ... rest as a parameter is only put at the end
   function desc(string, ...rest) {
     let str = "";
     for (let i = 0, len = rest.length; i < len; i++) {
       str += string[i] + rest[i];
     }
     str += string[string.length - 1];
     return str.toLocaleUpperCase();
   }
   let str = desc`My name is:${name},This year ${age}Year old`;
   console.log(str); // My name is XIAOHONG, 25 years old
  }

• Judgment string starts with a string

   let str = "hello world!";
   console.log(str.startsWith('h')); // true

• Judgment string ends with a string

   let str = "hello world!";
   console.log(str.endsWith('!')); // true

• Determine whether the character creation contains a string

   let str = "hello world!";
   console.log(str.includes('hello')); // true

• Duplicate string generation

   let str = "hello world!";
   console.log(str.repeat(3)); // hello world!hello world!hello world!