JavaScript Advanced Programming: Variables and Scope

1. Original value and reference value

The values ​​of the six simple data types are all primitive values. When a primitive value is assigned to another variable through a variable, a new value will be copied, and the two are independent of each other.

let num1 = 5
let num2 = num1

When a reference value is assigned to another variable through a variable, a value is also copied. This value is actually a pointer (reference), and the pointer still points to the same object.

let obj1 = new Object()

let obj2 = obj1

Since they point to the same reference object, adding properties to obj1 will also affect obj2 .

obj1.name = "zhangsan"

console.log(obj2.name) // zhangsan

When passing parameters to a function, there is only one situation where the parameters are passed by value, which is the same as assigning a variable. However, with reference values, the value passed is a pointer, but the pointer still points to the same object.

2. instanceof

typeof is generally suitable for judging primitive values, but not for reference values, because null and other objects return objects, and the specific type of the object is unknown. The best way to determine what type of object the reference value is is to use instanceof expression.

console.log(1 instanceof Object) //false
let obj = new Object();
console.log(obj instanceof Object) //true
console.log(null instanceof Object) //false



function fun(){
    //
}
console.log(fun instanceof Object) //true
console.log(fun instanceof Function) //true

console.log([] instanceof Object) //true
console.log([] instanceof Array) //true

3. Scope

The scope of a variable is called a scope or execution context. A variable is no longer visible outside of the scope. All systems have a scope chain when searching for a variable. First, look for the variable in the closest block scope that references it. If it is not found, continue looking in the outer local scope. If it is not found again, look in the global scope. If it is not found, an error ReferenceError: xxx is not defined will be reported.

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