What are the 4 ways to declare variables in javascript

Four ways to declare variables in JavaScript: 1. Use "var" to declare variables, such as "var a;"; 2. Use "function" to declare variables, such as "function Fun (num) {}"; 3. Use "let" to declare variables; 4. Use "const" to declare variables.

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

How to declare variables

There are several ways to declare variables in JavaScript:

• Before ES6, it was var and function
• New in ES6 Is adding let and const

function a way to declare variables?

Let’s verify it

Verification method one:

    function repeatFun (num) {
      return num;
    }

    repeatFun(10); // 10

    var repeatFun = 1;

    repeatFun(2); // Uncaught TypeError: repeatFun is not a function

This method uses var to repeatedly declare variables, but the latter will overwrite the former feature

Let’s take a look at what happened here:

• First, a function is declared, his name is repeatFun
• , and then called once, the result is 10
• After that, repeatFun is declared again with var, and initialized to 1
• The function repeatFun is called one last time
• The result is an error, content: repeatFun is not a function

According to the execution results, we can infer that there is a repeatFun variable in the browser's memory. It was a function before and was later redeclared by a var keyword and initially changed to 1.

Verification method two:

    {
      let repeatFun = 1;
      function repeatFun (num) {
        return num
      }
    }
    // Uncaught SyntaxError: Identifier 'repeatFun' has already been declared

The second method is to use a syntax of

ES6: using the feature of let that cannot be declared repeatedly to prove that function is also a declared variable The difference between

---

var, let and const:

• Variable declaration promotion

  • var has variable declaration promotion Functions can be used first and then declared, and vice versa.
  • let and const do not have the function of variable declaration promotion. They must be declared first before they can be used.

• Repeated declaration

  • var can be declared repeatedly, and the latter covers the former
  • let and const cannot be repeatedly declared

• scope The scope of

  • var's scope is bounded by functions
  • let and const are block scope
  • var can define global variables and local variables, let and const can only define local variables

• The special thing about const

  • cannot be modified after declaration (there are some differences in the behavior of reference types and basic types) Different)

---

Variable declaration promotion

• var has the function of variable declaration promotion, which can be used first and then declared
• let and const do not have the function of variable declaration promotion. They must be declared first before they can be used.

Example 1, verify var variable promotion:

var b = a + 1; // b： NaN 
var a = 1; // a： 1

First, declare it A variable b is initially recognized, and the initialized value is a 1 (what is the value of a?) <br/> Then a variable a is declared, and the initial recognition is 1 <br/> This is what the code looks like on the surface When doing these things, what is actually done is this:

• Every time a variable is declared, their declaration is placed at the top of the code, and they are all performed once Initialization, the value is: undefined, but the assignment position will not change,
• Then perform other operations

The following writing method can also achieve the same effect

var b;
var a;

b = a +1; // b： NaN 
a = 1; // a： 1

let and const behave differently from var

Example 2, verify whether there is variable promotion in let:

let b = a + 1; // Uncaught ReferenceError: a is not defined
let a = 1;

A scope error will be thrown directly when running. If you change it like this, There is no error:

let a = 1; // a： 1
let b = a + 1; // b： 2

const and let perform the same in terms of variable promotion

---

Duplicate declaration

• var can be declared repeatedly, and the latter overrides The former

• let and const cannot be declared repeatedly

  Example 1, verify the repeated declaration of var:

  var a = 1;
  var a = 2;
  var b = a + 1; // 3

  • First, declare the variable a, initialized as 1
  • Then declare variable a again, initialized to 2
  • Finally declare variable b, its initial value is a 1

  Example 2, verify the repetition of let Statement:

  let a = 1;
  let a = 2; // Uncaught SyntaxError: Identifier 'a' has already been declared

  var a = 1;
  let a = 2; //Uncaught SyntaxError: Identifier 'a' has already been declared

  • Obviously, variables declared using let in the same execution environment cannot be declared repeatedly, otherwise an error will be thrown <br/> const and let behave the same in terms of repeated declarations

---

The scope of the scope

• The scope of var is bounded by the function
• let and const are Block scope
• var can define global variables and local variables, let and const can only define local variables

encapsulates a factorial function as an example, without using tail recursion, just use for Implementation with if <br/> Example 1, factorial function verifies the scope:

  var num = 5;

  function factorial(num) {

      var result = 1，resultValue = 0;

      for (let i = num - 1; i >= 1; i--) {

        if (i === num - 1) {
          resultValue = num * i;
        }else{
          resultValue = num * i / num;
        }

        result *= resultValue;
      }

      // i 是用 let 进行定义它的作用域仅仅被限制在 for 循环的区域内
      // i++;// Uncaught ReferenceError: i is not defined

      return result;
  }

  // result 是用 var 进行定义，他的活动区域在 factorial 函数内
  // result++; // var的作用域.html:34 Uncaught ReferenceError: result is not defined

  factorial(num); // 120

const behaves the same as let in the scope

Example 2, verifies const Scope:

  {
    const NUM_1 = 10;
  }

  let b = NUM_1 + 1;  // Uncaught ReferenceError: NUM_1 is not defined

Example 3, verify that var can define global variables, let and const can only define local variables

  // 可以挂载到全局作用域上
  // var name = 'window scoped';

  let name = 'let scoped'; //是不挂载到全局作用域中

  let obj = {
    name: 'myName',
    sayName () {

       return function () {
        console.log(this.name); // 打印出来为空
      };
    }
  }

  obj.sayName()();
  console.log(window); //name 这个属性的值没有，如下图

若这样改一下就可以得到我们想要的值：

• 把用 var 定义的 name 的代码取消注释，把用 let 定义的 name 的代码注释。

这个同时也涉及到新问题 this 的指向。后面的文章再详细举例验证

---

const 的特殊之处

const 与 let , var 其实还是有些地方不一样的

例子1：验证 const 的特殊之处（一）<br/>

const NUM = 100; 
 NUM = 1000; // Uncaught TypeError: Assignment to constant variable

• 经过 const 方式进行声明，之后赋值完毕，则不可以进行改变，否则会报错

但是也有例外

例子二：验证 const 的特殊之处（二）

  const obj = {
    name: 'xiaoMing',
    sayName () {
      return this.name
    }
  };
  obj.sayName(); // xiaoMing

  obj.name = 'xiaoHong';
  obj.sayName(); // xiaoHong

• 使用 const 首先声明一个变量 obj , 并且这个变量指向我们在内存中创建的对象，你会发现我们改变里面的属性是没有任何问题

若这样改一下： <br/> 例子三：验证 const 的特殊之处（三）

  const obj = {
    name:'xiaoMing',
    sayName(){
      return this.name
    }
  };

  obj = {}; // Uncaught TypeError: Assignment to constant variable

• 若改变该变量的指向的对象，则就会报错。这种错误和 「 验证 const 的特殊之处（一）」的错误是一样的

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