JavaScript strict mode (use strict)
Overview
In addition to the normal running mode, ECMAscript 5 adds a second running mode: "strict mode". As the name suggests, this mode makes Javascript run under stricter conditions.
Why use strict mode
- Eliminate some unreasonable and lax aspects of Javascript syntax and reduce some weird behaviors;
- Eliminate some insecurities in code running and ensure the safety of code running;
- Improve compiler efficiency and increase running speed;
- Prepare for new versions of Javascript in the future Good foreshadowing.
"Strict Mode" reflects the more reasonable, safer and more rigorous development direction of Javascript. Mainstream browsers, including IE 10, already support it, and many large projects have begun to fully embrace it.
On the other hand, the same code may have different running results in "strict mode"; some statements that can be run in "normal mode" will not be able to be run in "strict mode" run. Mastering these contents will help you understand Javascript in more detail and make you a better programmer.
Enter flag
"use strict";
How to call
1. For a single script
<script>
"use strict";
console. log("This is strict mode.");
</script>
##2. For a single function
function strict(){
## "use strict";Return "This is strict mode.";
}
Function notStrict() {
Return "This is normal mode.";
}
## 3 Alternative ways of writing script files
(function (){
"use strict";
// some code here
})();
##Example 1: Example 2: Changes in syntax and behavior: 1 Explicit declaration of global variables In normal mode, if a variable is assigned a value without being declared, the default is a global variable. Strict mode prohibits this usage and global variables must be declared explicitly. "use strict"; v = 1; // Error, v is not declared ##for (i = 0; i < 2; i++) { // Error, i is not declared } "use strict"; var v = 1; with (o){ // Syntax error v = 2; } In normal mode, the scope of the eval statement depends on whether it is in the global scope or the function scope. In strict mode, the eval statement itself is a scope and can no longer generate global variables. The variables it generates can only be used inside eval. "use strict"; var x = 2; console.info(eval(" var x = 5; x")); // 5 console.info(x); // 2 3 Enhanced security measures (1) Prohibit this keyword from pointing to the global object function f(){ Return !this; } // Return false because "this" points to the global object, "! this" is false function f(){ "use strict"; return !this; } // Return true, because in strict mode, the value of this is undefined, so "!this" is true. (2) It is forbidden to traverse the call stack inside the function function f1(){ "use strict"; f1.caller; // Error report f1.arguments; // Error report } f1(); 4 Prohibited deletion of variables Cannot be deleted in strict mode variable. Only object properties with configurable set to true can be deleted. "use strict"; var x; delete x; // Syntax error var o = Object.create(null, {'x': { value: 1, configurable: true ##}}); delete o.x; // Delete successfully "use strict"; var o = {}; Object.defineProperty(o, "v", { value: 1, writable: false }); o.v = 2; // Error "use strict"; var o = { get v () { return 1; } }; o.v = 2; // Error reporting "use strict"; var o = {}; Object.preventExtensions(o) ; o.v = 1; // Error reporting "use strict"; delete Object.prototype; // Error "use strict"; var o = { p: 1, p: 2 ##}; // Syntax error "use strict"; function f(a, a, b) { // Syntax error Return ; } In normal mode, if the first digit of the integer is 0, it means that it is an octal number. For example, 0100 is equal to 64 in decimal. Strict mode prohibits this representation, the first bit of the integer is 0, and an error will be reported. var n = 0100; // Syntax error arguments is the parameter object of the function, and strict mode restricts its use. (1) Assignment of arguments is not allowed arguments++; // Syntax error var obj = { set p(arguments) { } }; // Syntax error try { } catch (arguments) { } // Syntax error function arguments() { } // Syntax error var f = new Function("arguments", "'use strict'; return 17;"); // Syntax error (2)arguments no longer tracks parameter changes function f(a) { a = 2; return [a, arguments[0]]; } f(1); // Normal mode is [2,2] function f(a) { "use strict"; ## a = 2; return [a, arguments[0]]; } f(1); // Strict mode is [2 ,1] "use strict"; var f = function() { return arguments.callee; }; f(); // Error "use strict"; if (true) { function f() { } / / Syntax error } for (var i = 0; i < 5; i++) { Function f2() { } // Syntax error } function package(protected) { // Syntax error "use strict"; } <!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
</head>
<body>
<p>浏览器按下 F12 开启调试模式，查看报错信息。</p>
<script>
"use strict";
x = 3.14; // 报错 (x 未定义)
</script>
</body>
</html>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>php中文网(php.cn)</title>
</head>
<body>
<p>浏览器按下 F12 开启调试模式，查看报错信息。</p>
<script>
x = 3.14; // 不报错
myFunction();
function myFunction() {
"use strict";
y = 3.14; // 报错 (y 未定义)
}
</script>
</body>
</html>
##7 Octal notation is prohibited
8 Restrictions on the arguments object
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