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Data types in JavaScript
Simple (basic) data types: Number, String, Boolean, Undefined, Null
Complex (reference) data types: Object , Array, Date, function, etc.
The following is an introduction to the difference between simple (basic) data types and complex (reference) data types:
Simple data types: store values directly in the stack
Complex data types: storing references on the stack
After understanding the storage methods of the above two data types, you can distinguish the difference between the two. You can do the following exercises:
var a =10; var b = a; //问:当改变a的值后,b的值是否发生改变 a=20; console.log(b); // 10
var s1 = new Object(); var s2 = s1; //问:改变s1的属性后,s2相同的属性是否改变 s1.name = "mh"; console.log(s2.name); //mh
function f2(arr) { arr = [9,8,7,6,5];//产生新的对象 arr[0]=-100; } var array = [1,2,4,7,5]; f2(array); console.log(array[0]);// 1
Variable promotion, function declaration and variable scope in JavaSript
First look at the following interview question:
var num = 10; fun(); function fun() { console.log(num); var num =20; }
Before learning, I would have directly answered 10, but now I know the concept of precompilation in JavaScript. The promotion concept of var keyword and the declaration concept of function know that the above code is equivalent to the following code:
var num;//全局作用域 遇到var和function 提升 function fun() { var num ; //局部做用户 遇到var 提升 console.log(num); num =20; } num = 10; fun();
Let’s look at the following question again:
//问题:为什么会出现下面的错误? //Uncaught TypeError:fnName is not a function console.log(fnName(1,2)); var fnName = function (a,b) { return a + b; }
The above code is equivalent to the following code:
var fnName; console.log(fnName(1,2)); fnName = function (a,b) { return a + b; }
You can see that because function is on the right side of the equal sign, only var will be promoted. As a result, the "Uncaught TypeError: fnName is not a function" error will be reported.
To understand the scope of variables, you can look at the following code:
f1(); console.log(c); console.log(b); console.log(a); function f1() { var a = b = c = 20; console.log(c); console.log(b); console.log(a); }
For continuous assignments such as var a=b=c=20;, they work locally Only a will be declared again within the scope, while the scopes of b and c are global scopes. Therefore, only a in the global scope reports the error "Uncaught ReferenceError: a is not defined".
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