javascript 常量_JavaScript中的常量

javascript 常量

JavaScript常数 (JavaScript Constants)

Before ES15, the only way to declare variables using the var keyword. JavaScript's inbuilt feature of hoisting variables could be carried out using the var keyword. If you're unfamiliar with variables in JavaScript, have a glance at Variables in JavaScript article on the website. If you wish to know what hoisting is, read through Hoisting in JavaScript.

在ES15之前，这是使用var关键字声明变量的唯一方法。 JavaScript的内置变量提升功能可以使用var关键字执行。如果您不熟悉JavaScript中的变量，请浏览网站上的JavaScript中的变量文章。如果您想知道什么是提升，请通读JavaScript中的提升。

Today we'll look at constants. Variables after ES15 could be declared in two ways - let and const. Before we dive deeper into const, let's understand what a constant is?

今天我们来看一下常量。 ES15之后的变量可以两种方式声明： let和const 。在深入研究const之前，让我们了解一个常数是什么？

Constants in most languages are something that retains their value during their block or wherever their life persists. In JavaScript, constants are values that we cannot modify later directly. This means if we declare a constant with a certain value, assigning some other value later in the program would result in an error.

大多数语言中的常量在其阻塞期间或生命持续存在的地方都可以保留其价值。在JavaScript中，常量是我们以后无法直接修改的值。这意味着，如果我们声明一个具有特定值的常数，则稍后在程序中分配其他一些值将导致错误。

    const a=10;
a=25;

Output

输出量

    Uncaught TypeError: Assignment to constant variable. at <anonymous>:1:2

We get a TypeError.

我们得到一个TypeError 。

We declare a constant using the keyword const.

我们使用关键字const声明一个常量。

const follows block scope. Consider the following,

const遵循块作用域。考虑以下，

var t=10;	//Value of t here is 10
{
const t=12;	//Value of t here is 12
}
//Value of t here is 10

Also, constants declared using the const keyword must be initialized and declared at the same time.

另外， 使用const关键字声明的常量必须同时初始化和声明 。

    const g=9.8;
//is correct, but
const g;
g=9.8i
//is incorrect.

However, an important catch to note that the const keyword is not what it looks like. The more accurate statement regarding const would be that it defines a constant reference to a value. This means that primitive values assigned to a variable using the const keyword cannot be altered but with objects, we are bound to no such restriction.

但是，需要注意的重要一点是const关键字不是它的外观。关于const的更准确的陈述是，它定义了对值的常量引用 。这意味着使用const关键字分配给变量的原始值不能更改，但是对于对象，我们不受任何限制。

If we declare an object with the const keyword, we can later change it by giving it new properties.

如果我们使用const关键字声明一个对象，我们以后可以通过为其赋予新属性来对其进行更改。

    const person={
Name: 'Fuzzy Sid',
Age: 20
}
person.gender='Male';

Adding a new property to a constant object does not yield us an error.

向常量对象添加新属性不会产生错误。

Similarly, we can change the elements of an array defined using the const keyword.

同样， 我们可以更改使用const关键字定义的数组的元素 。

    const arr=[1,2,3,4];
console.log(arr);       //[1,2,3,4]
arr[0]=0;
console.log(arr);       //[0,2,3,4]
arr.push_back(5);
console.log(arr);       //[0,2,3,4,5]

However, we cannot reassign new values to the array.

但是， 我们无法将新值重新分配给array 。

    arr = [9,8,7,6]

Output

输出量

Uncaught TypeError: Assignment to constant variable.at <anonymous>:1:4

Would give an error. But the trick around this would be to individually change the values of the array or maybe run a loop for the same.

会给出一个错误。但是，解决这个问题的技巧是单独更改数组的值，或者为该数组运行一个循环。

    for(let i=0,cnt=9; i<arr.length; i++,cnt--){
arr[i]=cnt;
}

Remember that const variables are not supported in versions of Internet Explorer before 10 and they are not hoisted. So you cannot use them before the declaration.

请记住，Internet Explorer 10之前的版本不支持const变量，也不会使用它们。因此，您不能在声明之前使用它们。