How to use JS strict mode

This time I will show you how to use JS strict mode and what are the precautions for using JS strict mode. The following is a practical case, let's take a look.

The so-called strict mode is actually a string that will not be assigned to any variable "use strict"

If this prompt is given in the global scope, the entire script will be in strict mode. You can also turn on strict mode only in functions

1. There are no global variables in strict mode

a="test"

An error will be reported in strict mode, but normal in non-strict mode

2. Delete variables

There are three declaration situations for var

• Global variables declared by var

• Local variables declared by var

• Global variables declared in eval()

The first and second cases cannot be deleted with delete.

First of all, although the global object declared in the first case is a property of windows, this property has configurable=false and therefore cannot be deleted

In the second case, local variables are even worse. I don’t even know who the attached object is. How to delete them

The third case eval() will be discussed later.

Back to topic. That is to say, in non-strict mode, you can delete variables, but it will fail and return false.

Deleting variables in strict mode will report an error.

3.Object

The operation object will report an error under the following circumstances

1. Assigning values ​​to read-only properties will report an error

2. Using delete for non-configurable properties will report an error

3. Adding properties to non-extensible objects will report an error

4. When using object literals, property names must be unique. For example

var person={
  name:"1",
  name:"2"
}

In non-strict mode, the second value will be taken by default, and in strict mode, an error will be reported.

4. Function

Strict mode requirementsFunction parametersnames must be unique

function（n,n）{
  // todo
}

In non-strict mode, this function declaration will not report an error. Only the second parameter can be accessed through the parameter name, and the first parameter must be accessed through arguments.

arguments are also different in the two modes

In non-strict mode, modifying the value of the named parameter will be reflected in the arguments object. In strict mode, the two values ​​​​are independent.

Arguments.callee (referencing the function itself) and arguments.caller (referencing the calling function) are eliminated.

In strict mode, function names cannot use jsreserved words

**In strict mode, functions can only be declared at the top level of the script and inside the function. Declaring the function in an if statement will cause a syntax error. **

if(true){
  function(){
   // 严格模式下报错
}
}

5.this

In non-strict mode

Use function 's apply(), call(), and the null or undefined value passed in will be converted into a global object. In strict mode, the this of a function is always the specified value, no matter what value is specified.

var a="1";
function test(){
 console.log(this.a)
}
test.call(null) 在非严格模式下会输出1，严格模式下this就指代的就是null,null没有a属性，就会报错。

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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