JS strict mode example explanation

This article mainly organizes relevant knowledge points and code examples in JS strict mode for everyone. If you are interested, follow the editor to learn together. I hope it can help everyone.

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

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

1. There are no global variables in strict mode

a="test"

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

2. Delete variables

var There are three declaration situations

• var Global variables declared in

• var Local variables declared in

• Global variables declared in eval()
  

The first and second cases cannot be deleted using 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.

The second case is local Variables are even worse. You don’t even know who the attached object is. How to delete them

The third case eval() will be discussed later.

Back to the 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

Under the following circumstances, an error will be reported when operating the object

1. Assigning a value to a read-only attribute will Error reporting

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"
}

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

4. Function

Strict mode requires that function parameter names must be unique

function（n,n）{
  // todo
}

In 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, both The values ​​are independent.

Eliminated arguments.callee (referencing the function itself) and arguments.caller (referencing the calling function).

In strict mode, function names cannot use js reserved words

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

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

5.this

Use function apply(), call(() in non-strict mode ), 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。

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