Strict model advantages:

Eliminate some unreasonable and imprecise aspects of Javascript grammar and reduce some unexpected situations.
Eliminate some unsafe aspects of code operation to ensure the safety of code operation;
Improving the efficiency of compiler and increasing the running speed;
Note that the same code may have different results in "strict mode"; some statements that can be run in "normal mode" will not run in "strict mode".

Restrictions on strict models:

Undeclared variables are not allowed to be used:

"use strict";
x = 3.14;                // Error reporting (x undefined)

"use strict";
x = {p1:10, p2:20};      // Error reporting (x undefined)

Deleting variables or objects is not allowed.

"use strict";
var x = 3.14;
delete x;                // Report errors

Deletion of functions is not allowed.

"use strict";
function x(p1, p2) {}; 
delete x;                // Report errors 

Variable renaming is not allowed:

"use strict";
function x(p1, p1) {};   // Report errors

Octal system is not allowed:

"use strict";
var x = 010;             // Report errors

Escaped characters are not allowed:

"use strict";
var x = \010;            // Report errors

It is not allowed to assign values to read-only attributes:

"use strict";
var obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});

obj.x = 3.14;            // Report errors

Assignment of an attribute read using the getter method is not allowed

"use strict";
var obj = {get x() {return 0} };

obj.x = 3.14;            // Report errors

It is not allowed to delete an attribute that is not allowed to delete:

"use strict";
delete Object.prototype; // Report errors

Variable names cannot use the "eval" string:

"use strict";
var eval = 3.14;         // Report errors

Variable names cannot use the arguments string:

"use strict";
var arguments = 3.14;    // Report errors

The following statements are not allowed to be used:

"use strict";
with (Math){x = cos(2)}; // Report errors

For some security reasons, variables created in scope eval() cannot be invoked:

"use strict";
eval ("var x = 2");
alert (x);               // Report errors

Prohibit this keyword from pointing to global objects.

function f(){
    return !this;
} 
// Return false, because "this" points to the global object, and "! This" is false.

function f(){ 
    "use strict";
    return !this;
} 
// Returns true, because in strict mode, this value is undefined, so "! This" is true.
//Therefore, if you forget to add new when using the constructor, this will no longer point to the global object, but will report an error.

function f(){
    "use strict";
    this.a = 1;
};
f();// Error reporting, this undefined

Keyword retention

In order to transition to a new version of Javascript in the future, some reserved keywords have been added to Strict Mode:

implements
interface
let
package
private
protected
public
static
yield
"use strict";
var public = 1500;      // Report errors

For more information: Newbie Course