What are the knowledge points that explain JS strict mode in detail?

This article has compiled relevant knowledge points and code examples in JS strict mode for everyone. If you are interested, let’s learn together.

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 use strict mode. You can also turn on strict mode only in the function

1. There are no global variables in strict mode

a="test"

There will be in strict mode Error is reported, normal in non-strict mode

2. Delete variable

var There are three declaration situations

• var declared Global variables

• var Local variables declared

• 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 an attribute of windows, this attribute has configurable=false and therefore cannot be deleted.

In the second case, local variables are even worse. No one knows how to delete the attached object

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 report an error

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

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

4. When using object literals, attribute 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 non-strict mode, this function declaration will not report an error. Pass The parameter name can only access the second parameter, 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.

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. Declaring the function in an if statement will cause a syntax error. . **

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

5.this

When using the function's apply(), call() in non-strict mode, the null or undefined value passed in will be converted to 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。

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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