javascript基礎學習一(變數)
阿新 • • 發佈:2018-12-10
var
用var申明一個變數:
var a = 1;
console.log(a) // 1
console.log(a) // undefined
var a = 1;
js的申明過程:
var a; // undefined,只申明,不賦值。會有個預設值undefined
a = 1 // 1
例子:
var a = 1;
var a;
console.log(a) // 1, 對重複定義且未賦值的變數,不會覆蓋之前的值
var a = 1;
var a = 2;
console.log(a) // 2, 重複定義有賦值,覆蓋之前的值
注:沒有修飾符的變數,為全域性變數,如:a = 3
,這裡a為全域性變數
let
let沒有與解析,不能先用再申明
console.log(a); // 報錯:a is not defined
let a = 2;
同一個作用域下不能重複申明,具有塊及作用域
let a = 1;
let a = 3;
console.log(a); // 報錯:Identifier 'a' has already been declared
let a = 1;
{
let a = 2;
console.log(a) // 2(不在同一個作用域)
}
const
同let,另外const申明後不可修改,物件除外
const a = 1; a = 2; console.log(a) // 報錯:Assignment to constant variable
const a = {
name: 'lyh',
age: 18
}
a.name = 'lyh2'
console.log(a) // {name: 'lyh2', age: 18}
const a = [1,2]
a[1] = 5
console.log(a)