Understanding strict mode in javascript_javascript tips

1. What is strict mode
The JavaScript code we usually write generally runs in the normal mode. In addition to the normal running mode, ECMAscript 5 adds a second running mode: "strict mode". As you can tell from the name, this mode will make JavaScript run in a more strict environment.
Major browsers, including IE 10, already support it, and many large projects have begun to fully embrace it. (Many projects on github use strict mode)
2. Enable strict mode
Enable strict mode for entire script
Put a specific statement "use strict";
before all statements Suppose there is a script reeoo.js, you can turn on strict mode like this:

"use strict";
var name = "Reeoo";
console.log(name);

There are natural pitfalls in this way of writing BUT. If we want to merge the code, I will now merge heigui.js:

heigui = "db";

Merge with reeoo.js. It is good for the two scripts to be executed separately, but an error will be reported when combined.
Uncaught ReferenceError: heigui is not defined(…)
The merging of a strict mode script and a non-strict mode script may cause the non-strict mode script code to report an error. It is recommended that the code be wrapped in an immediate execution function.

(function(){
 "use strict";
 var name = "reeoo";
})();

(function(){
 heigui = "db";
})();

In this way, no errors will be reported after merging.
Enable strict mode for a function
To enable strict mode for a function, you must place the "use strict"; statement before all statements in the function body.

function strictFun()
{
 // 函数级别严格模式语法
 'use strict';
 console.log('I am a strictmode function!');
}

function normalFun() { 
 console.log('I am a mormal function!');
}

Debugging strict mode in Chrome
I have this piece of code:

'use strict'
name = "reeoo";
console.log(name)

Paste this code directly into the Chrome console and execute it. Under normal circumstances, an error should be reported, but no error is reported.

Obviously, in strict mode, it is illegal for variables to be declared without var, but why is no error reported?
What the hell is this? Does Chrome not support strict mode? What a joke. . .
After searching online, it turns out that Chrome's console code runs in eval, and you cannot use strict mode for the eval function (it may not be completely correct, but what exactly Chrome does is unknown), as shown below Explain that the eval function can use strict mode:

To properly report errors in strict mode in the Chrome browser, you need to wrap an immediate execution function outside the code, or other similar measures.

(function(){
 'use strict'
 name = "reeoo";
 console.log(name) 
})()

That’s it
FireFox code scratch paper debugging strict mode
Chrome requires us to include a layer of closures to run strict mode. Since it is so troublesome, is there any other way to run strict mode code directly?
FireFox has a code scratch paper that can be run directly, the shortcut key SHIFT+F4

How strict is strict mode
Some important limitations in strict mode

1. Variable declaration
Using an undeclared variable is not allowed

"use strict";
name = "reeoo";

Error reporting (code draft paper, the same below)
Exception: ReferenceError: assignment to undeclared variable name

2. Modify the value of the read-only attribute

"use strict";
var testObj = Object.defineProperties({}, {
 prop1: {
 value: 10,
 writable: false // 一个只读的属性
 },
 prop2: {
 get: function () {
 }
 }
});
testObj.prop1 = 20; //尝试改变prop1的值
testObj.prop2 = 30;//尝试改变prop2的值

An error will be reported in strict mode:
Uncaught TypeError: Cannot assign to read only property 'prop1' of #